build: default to ppp 2.4.5 headers

2.4.5 has been out for a long time and fixes a number of bugs including
the DNS bug.  Use it.

configure.ac

@@ -84,7 +84,7 @@ AC_ARG_WITH([pppd-plugin-dir], AS_HELP_STRING([--with-pppd-plugin-dir=DIR], [pat
 if test -n "$with_pppd_plugin_dir" ; then
 	PPPD_PLUGIN_DIR="$with_pppd_plugin_dir"
 else
-	PPPD_PLUGIN_DIR="${libdir}/pppd/2.4.4"
+	PPPD_PLUGIN_DIR="${libdir}/pppd/2.4.5"
 fi
 AC_SUBST(PPPD_PLUGIN_DIR)