BUILD/MINOR: regex: missing header

When HAProxy is compiled with pcre, strlen() is used, but <string.h>
is not included.

This patch must be backported in 1.6
diff --git a/include/common/regex.h b/include/common/regex.h
index 30a5e3f..8a1703f 100644
--- a/include/common/regex.h
+++ b/include/common/regex.h
@@ -23,6 +23,7 @@
 #define _COMMON_REGEX_H
 
 #include <stdlib.h>
+#include <string.h>
 
 #include <common/config.h>