gnu: inkscape: New module

* gnu/packages/inkscape.scm,
  gnu/packages/patches/inkscape-stray-comma.patch: New files.
* gnu-system.am (GNU_SYSTEM_MODULES): Add inkscape.scm.
  (dist_patch_DATA): Add inkscape-stray-comma.patch.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>