Prevent inclusion of tsc.h when not needed

src/include/timestamp.h is an interface describing timestamp storage
in coreboot. Exporting this interface is complicated by inclusion of
tsc.h which is needed only for the API and is not used in structure
definitions. Including this dependency only when needed fixes the
problem.

Change-Id: Ie6b1460b1dab0f5b5781cb5a9fa89a1a52aa9f17
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: http://review.coreboot.org/1753
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>
diff --git a/src/include/timestamp.h b/src/include/timestamp.h
index 65bdf21..8bf5287 100644
--- a/src/include/timestamp.h
+++ b/src/include/timestamp.h
@@ -20,8 +20,6 @@
 #ifndef __TIMESTAMP_H__
 #define __TIMESTAMP_H__
 
-#include <cpu/x86/tsc.h>
-
 struct timestamp_entry {
 	uint32_t	entry_id;
 	uint64_t	entry_stamp;
@@ -55,6 +53,7 @@
 };
 
 #if CONFIG_COLLECT_TIMESTAMPS
+#include <cpu/x86/tsc.h>
 void timestamp_init(tsc_t base);
 void timestamp_add(enum timestamp_id id, tsc_t ts_time);
 void timestamp_add_now(enum timestamp_id id);