security: export security_kernel_load_data function

The firmware_loader can be built as a loadable module, which now
fails when CONFIG_SECURITY is enabled, because a call to the
security_kernel_load_data() function got added, and this is
not exported to modules:

ERROR: "security_kernel_load_data" [drivers/base/firmware_loader/firmware_class.ko] undefined!

Add an EXPORT_SYMBOL_GPL() to make it available here.

Fixes: 6e852651

 ("firmware: add call to LSM hook before firmware sysfs fallback")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: James Morris <james.morris@microsoft.com>