systemd: install udev.pc

systemd: install udev.pc

It provides udev, but doesn't install udev.pc, which causes other
recipes failed to figure out udevdir.

Fixed when systemd in DISTRO_FEATURES:
$ bitbake pcmciautils (or btrfs-tools):

Package udev was not found in the pkg-config search path.
Perhaps you should add the directory containing `udev.pc'
to the PKG_CONFIG_PATH environment variable
No package 'udev' found

Their udev rules file may not be installed according to each pkg's
implementation.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>