arm64: head: Convert install_el2_stub to SYM_INNER_LABEL

New assembly annotations have recently been introduced which aim to
make the way we describe symbols in assembly more consistent. Recently the
arm64 assembler was converted to use these but install_el2_stub was missed.

Signed-off-by: Mark Brown <broonie@kernel.org>
[catalin.marinas@arm.com: changed to SYM_L_LOCAL]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>