build: add ostree-soup-* to build process when configured with avahi

build: add ostree-soup-* to build process when configured with avahi

Avoid getting these link errors:
    ./.libs/libostree-1.so: undefined reference to `soup_uri_set_path'
    ./.libs/libostree-1.so: undefined reference to `soup_uri_new'
    ./.libs/libostree-1.so: undefined reference to `soup_uri_free'
    ./.libs/libostree-1.so: undefined reference to `soup_uri_set_scheme'
    ./.libs/libostree-1.so: undefined reference to `soup_uri_to_string'
    ./.libs/libostree-1.so: undefined reference to `soup_uri_set_host'
    ./.libs/libostree-1.so: undefined reference to `soup_uri_set_port'
    collect2: error: ld returned 1 exit status

Reproduce with:
    ./configure --with-avahi --without-soup

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Closes: #1666
Approved by: cgwalters