More errno.h fixes for OS X

* Because apparently the compiler isn't capable of giving more than one
  error at once :(.

renderdoc/serialise/rdcfile.cpp

@@ -23,6 +23,7 @@
  ******************************************************************************/
 
 #include "rdcfile.h"
+#include <errno.h>
 #include "3rdparty/stb/stb_image.h"
 #include "api/replay/version.h"
 #include "common/dds_readwrite.h"