plugin/core: autotools: add linking to xdot

plugin/core: autotools: add linking to xdot

E.g. plugin/core/gvrender_core_dot.c uses parseXDot from
lib/xdot/xdot.c.

Fixes these errors when building with Cygwin from the portable source
produced by the Ubuntu 18.03 runner in CI:

/cygdrive/c/GitLab-Runner/builds/magjac/graphviz/graphviz-2.49.1~dev.20210915.2014/plugin/core/gvrender_core_json.c:313: undefined reference to `parseXDot'
/usr/lib/gcc/x86_64-pc-cygwin/11/../../../../x86_64-pc-cygwin/bin/ld: /cygdrive/c/GitLab-Runner/builds/magjac/graphviz/graphviz-2.49.1~dev.20210915.2014/plugin/core/gvrender_core_json.c:331: undefined reference to `freeXDot'

For some reason, this error did not show up when building using a
build system generated on Cygwin with autogen.sh.