Use libgudev as regular dependency

Since eudev 3.1.3 libgudev is not shipped with this package,
so libgudev can be used as regular dependency regardless of
udev implementation selected.

Fixes following build error:
  http://autobuild.buildroot.net/results/320/320c052bda0f1b5afb1e5c83a7fb4dca6227c5a1//
  http://autobuild.buildroot.net/results/3b0/3b096f882030325b9290ad8860bd9cd373a11dc2//



Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>