via/cn700: Fix clang error with missing main

According to clang, main has no prototype for bcom/winnetp680
so add it into corresponding raminit.h

Change-Id: I8a55267901986757a4fa88ee13460ffbed3eeadc
Signed-off-by: Damien Zammit <damien@zamaudio.com>
Reviewed-on: https://review.coreboot.org/21356
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Martin Roth <martinroth@google.com>

src/northbridge/via/cn700/raminit.h

@@ -28,5 +28,6 @@ void ddr_ram_setup(const struct mem_controller *ctrl);
 
 /* mainboard specific */
 int spd_read_byte(unsigned device, unsigned address);
+void main(unsigned long bist);
 
 #endif