kselftest/arm64: Remove unneeded protype for validate_extra_context()

Nothing outside testcases.c should need to use validate_extra_context(),
remove the prototype to ensure nothing does.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220829160703.874492-6-broonie@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>