Partially revert "AMD Inagua: broadcom.c: Add missing prototype for `broadcom_init()`"

Commit 5d741567 added a prototype to broadcom.c to fix a warning.  This part
is fine.

It also changed mainboard.c to #include broadcom.c.  But broadcom.c is
already in Makefile.inc, now building will fail because the linker gets
broadcom_init() twice.

Undo the change to mainboard.c but keep the change to broadcom.c.

Change-Id: Ieccc098f477ffacccf4174056998034a220a9744
Signed-off-by: Jens Rottmann <JRottmann@LiPPERTembedded.de>
Reviewed-on: http://review.coreboot.org/3012
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Marc Jones <marc.jones@se-eng.com>

src/mainboard/amd/inagua/mainboard.c

@@ -26,8 +26,8 @@
 #include <device/pci_def.h>
 #include <southbridge/amd/sb800/sb800.h>
 #include "SBPLATFORM.h" 	/* Platfrom Specific Definitions */
-// #include "broadcom.c"	/* broadcom_init() */
 
+void broadcom_init(void);
 void set_pcie_reset(void);
 void set_pcie_dereset(void);