[gcov] Silence warning: comparison of integers of different signs

When building with Clang 11 on Windows, silence the following:

[432/5643] Building C object projects\compiler-rt\lib\profile\CMakeFiles\clang_rt.profile-x86_64.dir\GCDAProfiling.c.obj
F:\aganea\llvm-project\compiler-rt\lib\profile\GCDAProfiling.c(464,13): warning: comparison of integers of different signs: 'uint32_t' (aka 'unsigned int') and 'int' [-Wsign-compare]
    if (val != (gcov_version >= 90 ? GCOV_TAG_OBJECT_SUMMARY
        ~~~ ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.