Fix CentOS 6 build failure

CentOS 6 requires __STDC_LIMIT_MACROS to be defined before <stdint.h> is
included to expose INT32_MAX and others.

include/maxscale/cdefs.h

@@ -47,6 +47,13 @@
 # define __STDC_FORMAT_MACROS
 #endif
 
+/**
+ * Needs to be defined for INT32_MAX and others
+ */
+#ifndef __STDC_LIMIT_MACROS
+#define __STDC_LIMIT_MACROS
+#endif
+
 /**
  * Define intended for use with strerror.
  *