libxl: fix build with older glibc

libxl: fix build with older glibc

Commit 2b02882ebbbc "libxl: add infrastructure to track and query
'recent' domids" added a call to clock_gettime() into libxl. The man-
page for this states:

"Link with -lrt (only for glibc versions before 2.17)."

Unfortunately Centos 6 does have an glibc prior to that version, and the
libxl Makefile was not updated to add '-lrt' so the build will fail in
that environment.

This patch simply adds '-lrt' to LIBXL_LIBS unconditionally, as it does
no harm in newer environments.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Fixes: 2b02882ebbbc ("libxl: add infrastructure to track and query 'recent' domids")
Acked-by: Wei Liu <wl@xen.org>