gnu: Add kmplayer.

gnu: Add kmplayer.

`guix lint` reports two CVEs, both are unrelated:
- CVE-2018-5200: for vendor "pandora" and some 4.2.2.x version
- CVE-2019-9133: windows only (I assume it it alsow relates to the "pandora"
  vendor, since the version the CVE refers to as "solving the issue" does not
  exist at KDE.)

* gnu/packages/kde-multimedia.scm (kmplayer): New variable.
* gnu/packages/patches/kmplayer-aarch64.patch,
  gnu/packages/patches/kmplayer-upstream_Fix-build-with-Qt-5.9.patch: New
  files.
* gnu/local.mk: Add them.