Add a missing prototype to our libevent configure stanza.

Fixes bug 25474; bugfix on 0.3.2.5-alpha.

changes/bug25474

+  o Minor bugfixes (compilation):
+    - Fix a c99 compliance issue in our configuration script that was
+      causing compilation issues when compiling Tor with certain
+      versions of xtools. Fixes bug 25474; bugfix on 0.3.2.5-alpha.
+

configure.ac

@@ -631,7 +631,8 @@ TOR_SEARCH_LIBRARY(libevent, $trylibeventdir, [-levent $STATIC_LIBEVENT_FLAGS $T
 #include <winsock2.h>
 #endif
 struct event_base;
-struct event_base *event_base_new(void);],
+struct event_base *event_base_new(void);
+void event_base_free(struct event_base *);],
     [
 #ifdef _WIN32
 {WSADATA d; WSAStartup(0x101,&d); }