NuttX: fixes for printing size_t and int64_t

Added definition of PRId64 for C99 compatibility.
Used %zd for portable wat to print size_t.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/px4_defines.h

@@ -115,6 +115,9 @@ typedef param_t px4_param_t;
 #ifndef PRIu64
 #define PRIu64 "llu"
 #endif
+#ifndef PRId64
+#define PRId64 "lld"
+#endif
 
 /* 
  * POSIX Specific defines