do not define return type

When trying to install via panda it tries to precompile the module.
At this stage it thinks the sub would return a Str, so it breaks.
Inline::C should handle this some day, but trying now without return type.

lib/SDL/Surface.pm6

@@ -233,7 +233,7 @@ DLLEXPORT void *GetBuf( const void *from, void *to, size_t len )
 }
 '}
 
-our sub _get_pointer( int )  returns OpaquePointer  is inline('C')  {'
+our sub _get_pointer( int )  is inline('C')  {'
 DLLEXPORT void *GetPointer( size_t len )
 {
 	return (void *)len;