utils: resolve compiler warnings on implicit declarations

CC       osmo-auc-gen.o
	osmo-auc-gen.c: In function 'main':
	osmo-auc-gen.c:216:3: warning: implicit declaration of function
	'time' [-Wimplicit-function-declaration]

utils/osmo-auc-gen.c

@@ -25,6 +25,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include <string.h>
+#include <time.h>
 #include <getopt.h>
 #include <unistd.h>
 #include <inttypes.h>