firmware: smccc: Fix missing prototype warning for arm_smccc_version_init

firmware: smccc: Fix missing prototype warning for arm_smccc_version_init

Commit f2ae97062a48 ("firmware: smccc: Refactor SMCCC specific bits into
separate file") introduced the following build warning:

drivers/firmware/smccc/smccc.c:14:13: warning: no previous prototype for
	function 'arm_smccc_version_init' [-Wmissing-prototypes]
 void __init arm_smccc_version_init(u32 version, enum arm_smccc_conduit conduit)
             ^~~~~~~~~~~~~~~~~~~~~~

Fix the same by adding the missing prototype in arm-smccc.h

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/20200521110836.57252-1-sudeep.holla@arm.com
Signed-off-by: Will Deacon <will@kernel.org>