fix(main/mesa): Build with RTTI since LLVM now has it

[fornwall]

Fix the build error:

../src/meson.build:1808:8: ERROR:
Problem encountered: LLVM was built with RTTI, cannot build Mesa with RTTI disabled.
Remove cpp_rtti disable switch or use LLVM built without LLVM_ENABLE_RTTI.

[robertkirkman]

@fornwall sorry for delay, we are considering trying to turn -DLLVM_ENABLE_RTTI=ON back off again, because @truboxl has asked about it, see

• addpkg(x11/qt-creator): 18.0.0 #27309 (comment)

[fornwall]

@robertkirkman Ah, thanks for the info! Makes sense - closing this.