drm/msm: add extern C guard for the UAPI header

CRs-Fixed: 2038080
Change-Id: Idf66084b7d07c828d96538dfb7430da2fed796f2
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com> (over irc)
Git-commit: a62424e29dc33fdf1cf9efadfbf54deefbcfe7bf
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a62424e


Signed-off-by: Manoj Kumar AVM <manojavm@codeaurora.org>