Fix cmake non-explicit extension build warning (GIT8266O-787)

[rechrtb]

Fixes the warning:

-- Configuring done
CMake Warning (dev) at ESP8266_RTOS_SDK/tools/cmake/component.cmake:437 (add_library):
  Policy CMP0115 is not set: Source file extensions must be explicit.  Run
  "cmake --help-policy CMP0115" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  File:

    ESP8266_RTOS_SDK/components/spi_flash/src/partition.c
Call Stack (most recent call first):
  ESP8266_RTOS_SDK/components/spi_flash/CMakeLists.txt:11 (idf_component_register)
This warning is for project developers.  Use -Wno-dev to suppress it.

[HessianZ]

这个可以合啊。。。这么糙的bug

[CLAassistant]

All committers have signed the CLA.