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/lib/trace.c b/lib/trace.c
index f2402b9..6716c7c 100644
--- a/lib/trace.c
+++ b/lib/trace.c
@@ -5,6 +5,7 @@
 
 #include <common.h>
 #include <mapmem.h>
+#include <time.h>
 #include <trace.h>
 #include <asm/io.h>
 #include <asm/sections.h>