timestamp: fix incremental linking error for !HAVE_MONOTONIC_TIMER

In linking ramstage a single object file is created before linking
with the linker script. Though there is a weak timestamp_get() symbol
in timestamp.c any of its dependent symbols need to be available
during the incremental link. As not all platforms have
HAVE_MONOTONIC_TIMER enabled this will create a linking error.
Fix this by providing a hint to the compiler to remove dead code
and thus the dependent symbols causing linking errors in the presence
of !HAVE_MONOTONIC_TIMER.

Change-Id: Ib8a5dca2c12c2edac7605f403ed91b793823c8a3
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/10138
Tested-by: build bot (Jenkins)
Reviewed-by: Timothy Pearson <tpearson@raptorengineeringinc.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/lib/timestamp.c

@@ -180,6 +180,9 @@ uint64_t  __attribute__((weak)) timestamp_get(void)
 {
 	struct mono_time t1, t2;
 
+	if (!IS_ENABLED(CONFIG_HAVE_MONOTONIC_TIMER))
+		return 0;
+
 	mono_time_set_usecs(&t1, 0);
 	timer_monotonic_get(&t2);