Make udevlibexecdir be /lib

ie, fix another regression in our build system ; gentoo bug 455326

it's common for eg. mips systems to not have symlink from the directory
{lib32 or lib64} to /lib.  only the libraries belong to /$(get_libdir) and
everything in /lib/udev goes to, well, /lib/udev, NOT /$(get_libdir)/udev
     -- ssuominen

Signed-off-by: Ian Stakenvicius <axs@gentoo.org>