gst-plugins-base: add missing dependency on libxml2 for subparse

gst-plugins-base: add missing dependency on libxml2 for subparse

The gst-plugins-base configure script claims that the subparse plugin is
dependency-less - but it also silently disables subparse if libxml2 is
not available.  So depend on libxml2 when subparse is selected.

Note: the analogous gst1 subparse plugin does not have this requirement.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>