meson: disable graw tests on mingw

I can't figure out why symbols are being exposed that shouldn't.

v2: - change comment to FIXME

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Kristian H. Kristensen <hoegsberg@google.com>

src/gallium/tests/meson.build

@@ -25,4 +25,9 @@ endif
 if with_gallium_softpipe
   subdir('unit')
 endif
-subdir('graw')
+
+if host_machine.system() != 'windows' or cpp.get_id() != 'gcc'
+  # FIXME: This has linking errors I can't figure out with MinGW. works fine
+  # with MSVC, works fine with GCC on Linux.
+  subdir('graw')
+endif