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

Building the AMD Inagua board, the following warning is thrown.

        CC         mainboard/amd/inagua/get_bus_conf.ramstage.o
    src/mainboard/amd/inagua/broadcom.c:319:6: warning: no previous prototype for 'broadcom_init' [-Wmissing-prototypes]

This warning was introduced by commit 3926b4c5.

    commit 3926b4c520
    Author: Jens Rottmann <JRottmann@LiPPERTembedded.de>
    Date:   Fri Mar 1 19:41:41 2013 +0100

        AMD Inagua: add GEC firmware, document Broadcom BCM57xx Selfboot Patch format

        Reviewed-on: http://review.coreboot.org/2831

Adding the prototype to `broadcom.c` and removing it from
`mainboard.c` fixes the warning.

Change-Id: I1da0c4e972e129047dd8230d573f1c43fd71eb20
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/2952
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/mainboard/amd/inagua/broadcom.c

@@ -35,6 +35,8 @@
 #include <device/pci.h>
 #include <delay.h>
 
+void broadcom_init(void);
+
 #define be16(x)		cpu_to_be16(x)	//a little easier to type
 #define be(x)		cpu_to_be32(x)	//this is used a lot!
 

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);