soc/amd/common: Add included directory

If the symbol SOC_AMD_COMMON is selected, include the soc/amd/common
directory.  Until now this has been working due to the directory being
included as part of AGESA_INC in vendorcode.  That one is still
necessary in order to build the AGESA code so it is left in place
for now.

Change-Id: Ia8191897d2030c475c9268ae86faaf01952c6ace
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/21738
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/soc/amd/common/Makefile.inc

@@ -1,5 +1,7 @@
 ifeq ($(CONFIG_SOC_AMD_COMMON),y)
 
+CPPFLAGS_common += -I$(src)/soc/amd/common
+
 bootblock-y += agesawrapper.c
 bootblock-y += def_callouts.c
 bootblock-y += heapmanager.c