Build warning fix for AMD Family 12

This trivial change adds a prototype to an existing
header file to fix a build warning for the AMD family
12 cpus.

Change-Id: Ic666bfbef867d17607eaa0f59570aea987a31f93
Signed-off-by: Frank Vibrans <frank.vibrans@amd.com>
Signed-off-by: efdesign98 <efdesign98@gmail.com>
Reviewed-on: http://review.coreboot.org/218
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>

src/include/cpu/amd/amdfam12.h

@@ -48,6 +48,7 @@ void wait_all_other_cores_started(u32 bsp_apicid);
 void wait_all_aps_started(u32 bsp_apicid);
 void allow_all_aps_stop(u32 bsp_apicid);
 #endif
+void get_bus_conf(void);
 u32 get_initial_apicid(void);
 
 #endif /* CPU_AMD_FAM12_H */