Ensure that PRI formatting defines are available

On some systems, without __STDC_FORMAT_MACROS, the PRI defines
for printing 32 and 64 bit integers are not present.

include/maxscale/cdefs.h

@@ -39,6 +39,14 @@
 #undef OPENSSL_THREAD_DEFINES
 #define OPENSSL_THREAD_DEFINES 1
 
+/**
+ * Fix compile errors for PRId64
+ * in Centos 6
+ */
+#ifndef __STDC_FORMAT_MACROS
+# define __STDC_FORMAT_MACROS
+#endif
+
 /**
  * Define intended for use with strerror.
  *