pfring: workaround potential librt deps

It seems some version of pfring needs to be link with librt.

configure.ac

@@ -875,6 +875,9 @@
 
         LIBPFRING=""
         AC_CHECK_LIB(pfring, pfring_open,, LIBPFRING="no", [-lpcap])
+        if test "$LIBPFRING" = "no"; then
+            LIBPFRING=""
+            AC_CHECK_LIB(pfring, pfring_stats,, LIBPFRING="no", [-lpcap -lrt])
             if test "$LIBPFRING" = "no"; then
                 if test "x$enable_pfring" = "xyes"; then
                 echo
@@ -883,6 +886,9 @@
                 echo
                 exit 1
                 fi
+            else
+                LIBS="${LIBS} -lrt"
+            fi
         fi
 
         LIBPFRING_ENABLE_RING=""