Makefile.am: Fix missing files when generating tarball.

With

  $ ./configure --disable-gui
  $ make dist

currently a tarball would be produced, that lacks the .desktop file.
That happens, because an empty variable is appended to EXTRA_DIST.

The attached patch ensures release tarballs with complete, consistent
content and at the same time makes sure that with --disable-gui no .svg
files are installed.