Wrap libosxutils headers with "extern C"

Originally committed to SVN as r3118.

aegisub/src/subtitles_provider_libass.cpp

@@ -48,10 +48,11 @@
 #include <wx/filefn.h>
 
 #ifdef __APPLE__
+extern "C" {
 #include "libosxutil/libosxutil.h"
 #include <sys/param.h>
 #endif
-
+}
 
 ///////////////
 // Constructor