Fix test-udev.c to build and work on a uclibc system

Two issues are addressed: 1) Makefile.am has to provide config.h
information on HAVE_UNSHARE to test-udev.c.  2) MS_PRIVATE and
MS_REC are defined in <linux/fs.h> which must be directly included
on a uclibc system because of different header stackings.  Its
inclusion makes no difference on a glibc system.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
Signed-off-by: Ian Stakenvicius <axs@gentoo.org>