Add a preprocessor guard around __STDC_LIMIT_MACROS in IOBuf.cpp

Summary:
Adding the standard preprocessor guard around the `#define
__STD_LIMIT_MACROS` in IOBuf.cpp.

public

Reviewed By: knekritz

Differential Revision: D2825816

fb-gh-sync-id: 191824ec8d1f837e2b8380a2f8451972c421ceaa

folly/io/IOBuf.cpp

@@ -14,7 +14,9 @@
  * limitations under the License.
  */
 
+#ifndef __STDC_LIMIT_MACROS
 #define __STDC_LIMIT_MACROS
+#endif
 
 #include <folly/io/IOBuf.h>