kselftest/arm64: move get_header()

kselftest/arm64: move get_header()

Put this function in the header so that it can be used by other tests, without
needing to link to testcases.c.

This will be used by selftest/mm/protection_keys.c

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20240822151113.1479789-25-joey.gouly@arm.com
Signed-off-by: Will Deacon <will@kernel.org>