qt64-qtbase: fix building with 10.15 SDK

[tuffnatty]

Description

The 10.15 build reports:

/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_aqua_qt64/qt64-qtbase/work/qtbase-everywhere-src-6.4.3/src/gui/rhi/qrhimetal.mm:407:50: error: use of undeclared identifier 'MTLGPUFamilyApple7'
        caps.isAppleGPU = [d->dev supportsFamily:MTLGPUFamilyApple7];
                                                 ^

SDK 10.15 does not define this, so handle it like other such cases in the proximity: replace with a constant.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.14.6 18G9323 x86_64
Xcode 11.3.1 11C505

I could not test it on a 10.15 system, but I'm fairly sure the Buildbot will get further with this patch.

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@MarcusCalhoun-Lopez for port qt64.