Add proxy_module to the list of symbols

Add proxy_module to the list of symbols
detected by Netware export/import awk
script.

We do not export proxy_module by putting it
into modules/proxy/NWGNUproxy explicitely,
because all accompanying proxy modules need
this symbol to access their config. So let
the awk script add proxy_module to the imp
file and then export it from mod_proxy and
import it to all other mod_proxy_xxx modules
via the imp file.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1728780 13f79535-47bb-0310-9956-ffa450edef68