drivers/mrc_cache: Add missing include file to mrc_cache.h

Add region.h for use by the struct region_device * in the
mrc_cache_get_current() prototype.

BUG=b:69614064

Change-Id: I940beac45eb43e804bc84fead7d5337a1c4e2ac1
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/22921
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/include/mrc_cache.h

@@ -18,6 +18,7 @@
 
 #include <stddef.h>
 #include <stdint.h>
+#include <commonlib/region.h>
 
 enum {
 	MRC_TRAINING_DATA,