tests/xenstore: link in librt if necessary

tests/xenstore: link in librt if necessary

Old enough glibc has clock_gettime() in librt.so, hence the library
needs to be specified to the linker. Newer glibc has the symbol
available in both libraries, so make sure that libc.so is preferred (to
avoid an unnecessary dependency on librt.so).

Fixes: 93c9edbef51b ("tests/xenstore: Rework Makefile")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <iwj@xenproject.org>