tests: fix domaincapstest linking for libxl

tests: fix domaincapstest linking for libxl

Commit 11567cf added some libxl tests into domaincapstest and
added libvirt_driver_libxl_impl.la to domaincapstest_LDADD.

This causes link fail on systems without GNU regex implementation:

gmake[2]: Entering directory '/usr/home/novel/code/libvirt/tests'
  CCLD     domaincapstest
  ../src/.libs/libvirt_driver_libxl_impl.a(libvirt_driver_libxl_impl_la-libxl_capabilities.o):
  In function `libxlMakeCapabilities':
  libxl/libxl_capabilities.c:(.text+0x6b2): undefined reference to
  `rpl_regcomp'
  libxl/libxl_capabilities.c:(.text+0x6d0): undefined reference to
  `rpl_regerror'
  libxl/libxl_capabilities.c:(.text+0x803): undefined reference to
  `rpl_regexec'
  libxl/libxl_capabilities.c:(.text+0xa58): undefined reference to
  `rpl_regfree'
  clang-3.8: error: linker command failed with exit code 1 (use -v to
  see invocation)

This happens because on these system it tries to use gnulib's builtin
regex implementation, but doesn't link to gnulib.

Fix by adding $(GNULIB_LIBS) along with libvirt_driver_libxl_impl.la to
domaincapstest_LDADD.