media: atomisp: fix Wvisiblity warning

Some randconfig builds include ia_css_firmware.h without first
including linux/device.h:

In file included from atomisp/pci/mmu/sh_mmu_mrfld.c:23:
In file included from atomisp/pci/atomisp_compat.h:22:
In file included from atomisp/pci/atomisp_compat_css20.h:24:
In file included from atomisp/pci/ia_css.h:28:
In file included from atomisp/pci/ia_css_control.h:25:
drivers/staging/media/atomisp//pci/ia_css_firmware.h:52:29: error: declaration of 'struct device' will not be visible outside of this function [-Werror,-Wvisibility]

Add a forward declaration to avoid the warning.

Link: https://lore.kernel.org/linux-media/20210108082337.2305938-1-arnd@kernel.orgSigned-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

drivers/staging/media/atomisp/pci/ia_css_firmware.h

@@ -31,6 +31,8 @@ struct ia_css_fw {
 	unsigned int bytes; /** length in bytes of firmware data */
 };
 
+struct device;
+
 /* @brief Loads the firmware
  * @param[in]	env		Environment, provides functions to access the
  *				environment in which the CSS code runs. This is