changing to c name mangling, so that glgrab.c can use it

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_8_BRANCH@54029 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/mac/carbon/private.h

@@ -1229,7 +1229,7 @@ private:
 
 #ifdef __WXMAC_OSX__
 
-CGColorSpaceRef wxMacGetGenericRGBColorSpace(void);
+extern "C" CGColorSpaceRef wxMacGetGenericRGBColorSpace(void);
 void wxMacMemoryBufferReleaseProc(void *info, const void *data, size_t size);
 
 #endif