util: Fix unknown time_t types with musl-libc builds

Fixes the following build errors seen with musl-libc:

In file included from print.h:25:0,
                     from linreg.c:24:
util.h:364:32: error: unknown type name 'time_t'
int rate_limited(int interval, time_t *last);

Signed-off-by: Stephen Walker <stephendwalker+github@gmail.com>
[florian: extract patch from OpenWrt, add commit message]
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

util.h

@@ -20,6 +20,8 @@
 #ifndef HAVE_UTIL_H
 #define HAVE_UTIL_H
 
+#include <time.h>
+
 #include "ddt.h"
 #include "ether.h"