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

drivers/gpu/drm/nouveau/nvkm/core/firmware.c

@@ -20,6 +20,7 @@
  * DEALINGS IN THE SOFTWARE.
  */
 #include <core/device.h>
+#include <core/firmware.h>
 
 /**
  * nvkm_firmware_get - load firmware from the official nvidia/chip/ directory