Add mako dependency

[chsasank]

Without this, the build fails with following error:

Consolidate compiler generated dependencies of target obj.clangFormat                                                                                                                                              
Consolidate compiler generated dependencies of target obj.clangToolingInclusions                                                                                                                                   
Traceback (most recent call last):                                                                                                                                                                                 
  File "/home/sasank/code/igc_workspace/igc/IGC/GenISAIntrinsics/generator/generate_intrinsic_files.py", line 11, in <module>                                                                                      
    from Intrinsic_utils import file_path, dir_path                                                                                                                                                                
  File "/home/sasank/code/igc_workspace/igc/IGC/GenISAIntrinsics/generator/Intrinsic_utils.py", line 14, in <module>                                                                                               
    from mako.template import Template                                                                                                                                                                             
ModuleNotFoundError: No module named 'mako'                                                                                                                                                                        
Consolidate compiler generated dependencies of target obj.clangToolingCore                                                                                                                                         
Consolidate compiler generated dependencies of target obj.clangRewrite
Consolidate compiler generated dependencies of target obj.clangCrossTU
make[2]: *** [IGC/GenISAIntrinsics/CMakeFiles/GenISAIntrinsics.dir/build.make:84: IGC/Release/GenIntrinsicDescription.h] Error 1
make[1]: *** [CMakeFiles/Makefile2:33415: IGC/GenISAIntrinsics/CMakeFiles/GenISAIntrinsics.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....

So added instructions to install mako into docs.

[pszymich]

Hi, we process the PRs internally and a recent documentation commit has integrated some or all of your suggestions. You've been added as a coauthor. f13266a
Thanks!

[eero-t]

@pszymich Why docs now state that user should install mako with pip:
https://github.com/intel/intel-graphics-compiler/blob/master/documentation/build_ubuntu.md

When e.g. IGC's own build script for Ubuntu uses python3-mako distro package for it:
https://github.com/intel/intel-graphics-compiler/blob/master/scripts/buildIGC.sh#L52
?