qemu-i440fx: add a prototype for main()

This probably belongs elsewhere, but I haven't found a nice place yet.

Change-Id: I9ca52db33905cf4ee229d7ff44012105915271a8
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/4720
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

src/mainboard/emulation/qemu-i440fx/romstage.c

@@ -33,6 +33,7 @@
 
 #include "memory.c"
 
+void main(unsigned long bist);
 void main(unsigned long bist)
 {
 	int cbmem_was_initted;