check: add missing harness function to symbol export list

Fixes in user code:

undefined reference to `gst_harness_add_element_sink_pad'

Also reorder harness function list to be strictly in alphabetical order and
double check the list with:

awk '{ if ($1 !~ /#define/) if ($2 ~ /gst_harness_/) { print $2 }; if ($3 ~ /gst_harness_/) { print $3} }' libs/gst/check/gstharness.h | sort
1 file changed