Add missing catch_platform include to compiler capabilities

This was removed in 64be2ad, to fix OS X approval tests. At the time
I couldn't investigate because I didn't have access to OS X, but this
fixed it (and since we don't have MinGW in CI, the breakage went
unnoticed).

As it turns out, piece-wise compilation of the Compact
reporter had broken OS X detection for a long time, and fixing it
was what broke the approvals. After the approval scripts were
changed to compensate, this change passes approval tests and fixes

include/internal/catch_compiler_capabilities.h

@@ -24,6 +24,7 @@
 // Many features, at point of detection, define an _INTERNAL_ macro, so they
 // can be combined, en-mass, with the _NO_ forms later.
 
+#include "catch_platform.h"
 
 #ifdef __cplusplus