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/net/nfs.c b/net/nfs.c
index aca0ca5..97e62f1 100644
--- a/net/nfs.c
+++ b/net/nfs.c
@@ -33,6 +33,7 @@
 #include <mapmem.h>
 #include "nfs.h"
 #include "bootp.h"
+#include <time.h>
 
 #define HASHES_PER_LINE 65	/* Number of "loading" hashes per line	*/
 #define NFS_RETRY_COUNT 30