component/avahi: Add libexpat.

Avahi was not buildable on Debian 11.

component/avahi/buildout.cfg

@@ -30,6 +30,6 @@ configure-options =
   --with-systemdsystemunitdir=no
 environment =
   PATH=${pkgconfig:location}/bin:${glib:location}/bin:%(PATH)s
-  CFLAGS=-I${gdbm:location}/include
-  LDFLAGS=-L${gettext:location}/lib -Wl,-rpath=${gettext:location}/lib -L${zlib:location}/lib -Wl,-rpath=${zlib:location}/lib -L${glib:location}/lib -Wl,-rpath=${glib:location}/lib -L${gdbm:location}/lib -Wl,-rpath=${gdbm:location}/lib
-  PKG_CONFIG_PATH=${glib:location}/lib/pkgconfig:${pcre:location}/lib/pkgconfig:${dbus:location}/lib/pkgconfig
+  CFLAGS=-I${gdbm:location}/include -I${libexpat:location}/include
+  LDFLAGS=-L${gettext:location}/lib -Wl,-rpath=${gettext:location}/lib -L${zlib:location}/lib -Wl,-rpath=${zlib:location}/lib -L${glib:location}/lib -Wl,-rpath=${glib:location}/lib -L${gdbm:location}/lib -Wl,-rpath=${gdbm:location}/lib -L${libexpat:location}/lib -Wl,-rpath=${libexpat:location}/lib
+  PKG_CONFIG_PATH=${glib:location}/lib/pkgconfig:${pcre:location}/lib/pkgconfig:${dbus:location}/lib/pkgconfig:${libexpat:location}/lib/pkgconfig