unfs3: correct configure option

On some new distro like ubuntu21.04, unfs3-native compile failed with
error: undefined reference to `xdr_uint32', since new distro has new
glibc.

>From glibc 2.27 rpc support is dropped, so unfs3 need to link to
libtirpc.

Here is defination of ac_link:
ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'

Depended library should be added into LIBS, not LDFLAGS, otherwise,
gcc may not load the lib since it is before conftest.$ac_ext during
configure. Finally, it results in compile failed.

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 27867862)
Signed-off-by: Steve Sakoman <steve@sakoman.com>