progdocs: Exclude ./third-party from documentation generation

We don't need pjproject's documentation embedded in Asterisk's.

Change-Id: Iea6f5a621c0f4e3168dda3321eaab258d9f24a17

doc/asterisk-ng-doxygen.in

@@ -616,7 +616,8 @@ RECURSIVE              = yes
 EXCLUDE                = doc/api \
                          menuselect \
                          res/pjproject \
-                         addons/ooh323c/src
+                         addons/ooh323c/src \
+                         third-party
 
 # The EXCLUDE_SYMLINKS tag can be used select whether or not files or
 # directories that are symbolic links (a Unix filesystem feature) are excluded