helper: util: include util.h

Let source file to include its file .h to validate the exported
prototypes.

Detected through 'sparse' tool.

Change-Id: I5de107b4f8a468f0e37f06171f5f0c3c0546db1a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7596
Tested-by: jenkins

src/helper/util.c

@@ -12,6 +12,7 @@
 
 #include "log.h"
 #include "time_support.h"
+#include "util.h"
 
 COMMAND_HANDLER(handler_util_ms)
 {