configure.ac: fix build with --enable-bufferevents

When building with bufferevents enabled, configure.ac throws an
error if "$ac_cv_header_event2_bufferevent_ssl_h" is not set to
"yes".  However, nowhere was AC_CHECK_HEADERS(event2/bufferevent_ssl.h)
done.  This commit adds the check.

configure.ac

@@ -454,7 +454,7 @@ AC_CHECK_MEMBERS([struct event.min_heap_idx], , ,
 [#include <event.h>
 ])
 
-AC_CHECK_HEADERS(event2/event.h event2/dns.h)
+AC_CHECK_HEADERS(event2/event.h event2/dns.h event2/bufferevent_ssl.h)
 
 LIBS="$save_LIBS"
 LDFLAGS="$save_LDFLAGS"