Broken pyerl build

Small patch to fix following error. erl_interface paths should be included to `plugins/pyerl/uwsgiconfig.py`

       [~/pg/uwsgi]$ python uwsgiconfig.py --build pyerl
       .........skipped........
       [thread 3][gcc -pthread] plugins/pyerl/pyerl.o
       In file included from plugins/pyerl/pyerl.c:1:0:
       plugins/pyerl/../erlang/erlang.h:1:16: fatal error: ei.h: No such file or directory
       compilation terminated.

Ubuntu 12.10, Python 2.7, esl-erlang 1:15.b.3-2