cli,output: include string.h for strlen()

This patch fixes the following compiler warning:

mmcli-output.c:783:19: error: implicitly declaring library function 'strlen' with type 'unsigned long (const char *)' [-Werror,-Wimplicit-function-declaration]
            aux = strlen (section_infos[field_infos[item_l->field].section].name);
                  ^

cli/mmcli-output.c

@@ -19,6 +19,7 @@
  */
 
 #include <stdio.h>
+#include <string.h>
 
 #include <libmm-glib.h>
 #include "mm-common-helpers.h"