coccinelle: meson: rename variables to be more specific

coccinelle: meson: rename variables to be more specific

In Meson, included subdirs export their variables to top level Meson
builds. In 'contrib/coccinelle/meson.build', we define two such
variables `sources` and `headers`. While these variables are specific to
the checks in the 'contrib/coccinelle/' directory, they also pollute the
top level 'meson.build'.

Rename them to be more specific, this ensures that they aren't
mistakenly used in the upper levels and avoid variable name collisions.

While here, change the empty list denotation to be consistent with other
places.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>