firmware: arm_ffa: Declare ffa_bus_type structure in the header

smatch reports:
drivers/firmware/arm_ffa/bus.c:108:17: warning:
	symbol 'ffa_bus_type' was not declared. Should it be static?

ffa_bus_type is exported to be useful in the FF-A driver. So this
warning is not correct. However, declaring the ffa_bus_type structure
in the header like many other bus_types do already removes this warning.
So let us just do the same and get rid of the warning.

Link: https://lore.kernel.org/r/20231024105715.2369638-1-sudeep.holla@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

include/linux/arm_ffa.h

@@ -209,6 +209,8 @@ bool ffa_device_is_valid(struct ffa_device *ffa_dev) { return false; }
 #define module_ffa_driver(__ffa_driver)	\
 	module_driver(__ffa_driver, ffa_register, ffa_unregister)
 
+extern struct bus_type ffa_bus_type;
+
 /* FFA transport related */
 struct ffa_partition_info {
 	u16 id;