Avoid errors about re-defining __STDC_FORMAT_MACROS

Bug: skia:
Change-Id: I880e3d5a668743ac12fb0101baca637443e920b4
Reviewed-on: https://skia-review.googlesource.com/33082
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/utils/SkJSONWriter.cpp b/src/utils/SkJSONWriter.cpp
index 3b92aa4..430c802 100644
--- a/src/utils/SkJSONWriter.cpp
+++ b/src/utils/SkJSONWriter.cpp
@@ -6,7 +6,10 @@
  */
 
 // Make sure that the PRI format string macros are defined
+#ifndef __STDC_FORMAT_MACROS
 #define __STDC_FORMAT_MACROS
+#endif
+
 #include <inttypes.h>
 #include <stdarg.h>