xmldoc: Avoid whitespace around value for parameter/required.

Otherwise, the value 'false' was not found in the enumerated set of
the XML DTD for the XML attribute 'required' in the XML element
'parameter'. Therefore, DTD validation of the runtime XML failed.

ASTERISK-29790

Change-Id: Id13f230ad65a70dd8c2e3ae9ac85d1e841aed03e