inttypes.h is built-in header file since MSVC 2013

The reason is that the types defined in libgit2's inttypes.h collide with system inttypes.h
3rd party library header files may directly reference MSVC's built-in inttypes.h

Fixes #3476

include/git2/common.h

@@ -20,7 +20,7 @@
 # define GIT_END_DECL	/* empty */
 #endif
 
-#ifdef _MSC_VER
+#if defined(_MSC_VER) && _MSC_VER < 1800
  GIT_BEGIN_DECL
 # include "inttypes.h"
  GIT_END_DECL