common: Move get_ticks() function out of common.h

This function belongs in time.h so move it over and add a comment.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/common/bootretry.c b/common/bootretry.c
index 47aaaa8..dac891f 100644
--- a/common/bootretry.c
+++ b/common/bootretry.c
@@ -9,6 +9,7 @@
 #include <cli.h>
 #include <env.h>
 #include <errno.h>
+#include <time.h>
 #include <watchdog.h>
 
 #ifndef CONFIG_BOOT_RETRY_MIN