util/cbmem: fix an unused parameter issue in timestamp_get

Fix an unused parameter error when building on devices where __i386__
and __x86_64__ are not defined.

BUG=none
TEST=none

Change-Id: I6c04c8e7b931565c87d358aac1025ebcb7617b13
Signed-off-by: Nick Vaccaro <nvaccaro@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63880
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/cbmem/cbmem.c

@@ -559,6 +559,8 @@ static uint64_t timestamp_get(uint64_t table_tick_freq_mhz)
 
 	if (tsc_freq_khz)
 		return tsc * table_tick_freq_mhz * 1000 / tsc_freq_khz;
+#else
+	(void)table_tick_freq_mhz;
 #endif
 	die("Don't know how to obtain timestamps on this platform.\n");
 	return 0;