BUILD/MINOR: missing header file

In the header file "common/regex.h", the C keyword NULL is used. This
keyword is referenced into the header file "stdlib.h", but this is not
included.
diff --git a/include/common/regex.h b/include/common/regex.h
index 0104019..1cc471b 100644
--- a/include/common/regex.h
+++ b/include/common/regex.h
@@ -22,6 +22,8 @@
 #ifndef _COMMON_REGEX_H
 #define _COMMON_REGEX_H
 
+#include <stdlib.h>
+
 #include <common/config.h>
 
 #ifdef USE_PCRE