firmware: qcom_scm: Add missing extern specifier

Commit 3a99f121 ("firmware: qcom: scm: Introduce pas_metadata
context") left out the `extern` specifier for the API it introduced, so
add it.

Signed-off-by: Guru Das Srinagesh <quic_gurus@quicinc.com>
Link: https://lore.kernel.org/r/bce25c8e215f7cfc7b0780d6965d09f5efe1cc5f.1690503893.git.quic_gurus@quicinc.com


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