scanner: move include directives before extern "C" wrapper

scanner: move include directives before extern "C" wrapper

Adding extern "C" wrapper before includes (especially system ones) is
illadvised as the headers themselves can behave diffently in that case.

See the section "Including C Headers in C++ Code" in the following
http://www.oracle.com/technetwork/articles/servers-storage-dev/mixingcandcpluspluscode-305840.html

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Quentin Glidic <sardemff7+git@sardemff7.net>