Without this, make in sys/modules/gpio generates this error:
===> gpiospi (all)
Warning: Object directory not changed from original /home/lwhsu/freebsd-head/sys/modules/gpio/gpiospi
cc -O2 -pipe -fno-strict-aliasing -Werror -D_KERNEL -DKLD_MODULE -nostdinc -I. -I/home/lwhsu/freebsd-head/sys/modules/gpio/gpiospi/../../../dev/gpio/ -I. -I/home/lwhsu/freebsd-head/sys -fno-common -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer -MD -MF.depend.gpiospi.o -MTgpiospi.o -mcmodel=kernel -mno-red-zone -mno-mmx -mno-sse -msoft-float -fno-asynchronous-unwind-tables -ffreestanding -fwrapv -fstack-protector -Wall -Wredundant-decls -Wnested-externs -Wstrict-prototypes -Wmissing-prototypes -Wpointer-arith -Winline -Wcast-qual -Wundef -Wno-pointer-sign -D__printf__=__freebsd_kprintf__ -Wmissing-include-dirs -fdiagnostics-show-option -Wno-unknown-pragmas -Wno-error-tautological-compare -Wno-error-empty-body -Wno-error-parentheses-equality -Wno-error-unused-function -Wno-error-pointer-sign -Wno-error-shift-negative-value -mno-aes -mno-avx -std=iso9899:1999 -c /home/lwhsu/freebsd-head/sys/modules/gpio/gpiospi/../../../dev/gpio//gpiospi.c -o gpiospi.o
In file included from /home/lwhsu/freebsd-head/sys/modules/gpio/gpiospi/../../../dev/gpio//gpiospi.c:45:
/home/lwhsu/freebsd-head/sys/dev/gpio/gpiobusvar.h:33:10: fatal error: 'opt_platform.h' file not found
#include "opt_platform.h"
^
1 error generated.
*** Error code 1