drm/nouveau/core: add missing header dependencies

We get 2 warnings when building kernel with W=1:
drivers/gpu/drm/nouveau/nvkm/core/firmware.c:34:1: warning: no previous prototype for 'nvkm_firmware_get' [-Wmissing-prototypes]
drivers/gpu/drm/nouveau/nvkm/core/firmware.c:58:1: warning: no previous prototype for 'nvkm_firmware_put' [-Wmissing-prototypes]

In fact, both functions are declared in
drivers/gpu/drm/nouveau/include/nvkm/core/firmware.h,
so this patch adds missing header dependencies.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>