env-util: make env-util.h self contained

The header file references strlen(), hence it should include string.h

src/basic/env-util.h

@@ -6,6 +6,7 @@
 #include <stdio.h>
 
 #include "macro.h"
+#include "string.h"
 
 bool env_name_is_valid(const char *e);
 bool env_value_is_valid(const char *e);