sb/amd/pi/hudson: fix gpio.h and smi.h include location

This fixes the following compile error when trying to build the APU2
board with HAVE_SMI_HANDLER selected and the NO_SMM select removed:

In file included from src/soc/amd/common/block/gpio/gpio.c:8:
src/include/gpio.h:6:10: fatal error: soc/gpio.h: No such file or directory
    6 | #include <soc/gpio.h> /* IWYU pragma: export */
      |          ^~~~~~~~~~~~

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: Ie06044b12f5cbcc55a2706ec566afd2eb294c62b
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79846
Reviewed-by: Matt DeVillier <matt.devillier@amd.corp-partner.google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/southbridge/amd/pi/hudson/Makefile.inc

@@ -36,6 +36,8 @@ all-y += reset.c
 smm-y += smihandler.c
 smm-y += smi_util.c
 
+CPPFLAGS_common += -I$(src)/southbridge/amd/pi/hudson/include
+
 # ROMSIG At ROMBASE + 0x20000:
 # +-----------+---------------+----------------+------------+
 # |0x55AA55AA |EC ROM Address |GEC ROM Address |USB3 ROM    |