Register the BP_XProfile_Field_Type in bp_xprofile_get_field_types()

This bit was omitted from r8632, where the URL field type was introduced.

See #5501

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8634 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Pinking shears

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8635 cdf35c40-ae34-48e0-9cc9-0c9da1808c22