meson: hide warnings from external project `gtest`

gtest is an external project that is copied in this tree for technical
reasons, but isn't maintained by us, so its warnings are irrelevant.

Cc: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

src/gtest/meson.build

@@ -22,6 +22,7 @@ libgtest = static_library(
   'gtest',
   files('src/gtest-all.cc', 'src/gtest_main.cc'),
   include_directories : include_directories('include'),
+  cpp_args : '-w',
   build_by_default : false,
 )