Enable -Woverload-virtual for Mac OS X, too

...after f29e959c "enable -Woverloaded-virtual
gcc warning" had already enabled it for unxgcc.mk a long time ago.

Change-Id: Ic33e026a3d7200c779cec0cd9f23058debc0c74e

solenv/gbuild/platform/macosx.mk

@@ -49,6 +49,7 @@ gb_CFLAGS := \
 gb_CXXFLAGS := \
 	$(gb_CXXFLAGS_COMMON) \
 	-fPIC \
+	-Woverloaded-virtual \
 	-Wno-ctor-dtor-privacy \
 	-fno-strict-aliasing \
 	-fsigned-char \