Rename dialplan_functions.xml to dialplan_functions_doc.xml

When using COMPILE_DOUBLE, dialplan_functions.xml is mistaken
for the source for an embedded XML document and gets compiled
to dialplan_functions.o.  This causes dialplan_functions.c to
be ignored making its functions unavailable and causing chan_pjsip
to fail to load.

diff --git a/channels/pjsip/dialplan_functions.xml b/channels/pjsip/dialplan_functions_doc.xml
similarity index 100%
rename from channels/pjsip/dialplan_functions.xml
rename to channels/pjsip/dialplan_functions_doc.xml