mainboard/apple/macbook21/romstage.c: Missing prototype header

Fix warning thrown by Clang due to missing prototype for main
entry point function in -ffreestanding. main() is as any other
function in freestanding and so a prototype is strictly needed.

Change-Id: Ic27e0f93065b1aa85d3979db61b5e2ff0dd2a310
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7518
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/mainboard/apple/macbook21/romstage.c

@@ -27,6 +27,7 @@
 #include <arch/io.h>
 #include <device/pci_def.h>
 #include <device/pnp_def.h>
+#include <cpu/intel/romstage.h>
 #include <cpu/x86/lapic.h>
 #include <lib.h>
 #include <cbmem.h>