Define __STDC_FORMAT_MACROS before inttypes.h.

It seems Android's inttypes.h refuses to define those macros on C++ unless
__STDC_FORMAT_MACROS is set. This unbreaks the roll on Android.

Change-Id: Iad6c971b4789f0302534d9e5022534c6124e0ff0
Reviewed-on: https://boringssl-review.googlesource.com/4202
Reviewed-by: Adam Langley <agl@google.com>