firmware: qcom: scm: remove unneeded 'extern' specifiers

'extern' specifiers do nothing for function declarations. Remove them
from the private qcom-scm header.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Tested-by: Andrew Halaney <ahalaney@redhat.com> # sc8280xp-lenovo-thinkpad-x13s
Link: https://lore.kernel.org/r/20231017092732.19983-4-brgl@bgdev.pl


Signed-off-by: Bjorn Andersson <andersson@kernel.org>