package/x11r7/xdriver_xf86-input-vmmouse: Add optional dependency to udev

package/x11r7/xdriver_xf86-input-vmmouse: Add optional dependency to udev

Udev support is enabled by default
http://cgit.freedesktop.org/xorg/driver/xf86-input-vmmouse/tree/configure.ac#n113

and only disabled on demand
http://cgit.freedesktop.org/xorg/driver/xf86-input-vmmouse/tree/configure.ac#n115

Fixes
http://autobuild.buildroot.net/results/201/2011ae075db2cc3c22706f96b190de7a4bda99eb/
http://autobuild.buildroot.net/results/cd3/cd3730ed0272c3f335ee90e0cbc4027b07945db3/
http://autobuild.buildroot.net/results/7f7/7f79ca6a35f405b56aa2d619a649ffb367da7790/
http://autobuild.buildroot.net/results/7c9/7c9ab14b22bb14ce70905d3d1471d96241242827/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>