package/webkitgtk: add optional systemd dependency

package/webkitgtk: add optional systemd dependency

- systemd support/USE_SYSTEMD option was added since 2.30.0,
  so add an optional dependency

Fixes:

  -- Could NOT find Systemd (missing: Systemd_LIBRARY Systemd_INCLUDE_DIR)
  CMake Error at Source/cmake/OptionsGTK.cmake:425 (message):
    libsystemd is needed for USE_SYSTEMD

Reported-by: C Larbi <pkl2000us@gmail.com>
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>