BUILD: pattern: include errno.h

Commit 3c79d4bdc introduced the use of errno in pattern.c without
including errno.h.
If we build haproxy without any option errno is not defined and the
build fails.

src/pattern.c

@@ -12,6 +12,7 @@
 
 #include <ctype.h>
 #include <stdio.h>
+#include <errno.h>
 
 #include <common/config.h>
 #include <common/standard.h>