MINOR: stick-tables: Adds support for new "gpc1" and "gpc1_rate" counters.

Implement exactly the same code as this has been done for "gpc0" and "gpc0_rate"
counters.
diff --git a/doc/peers-v2.0.txt b/doc/peers-v2.0.txt
index 767721e..6812c85 100644
--- a/doc/peers-v2.0.txt
+++ b/doc/peers-v2.0.txt
@@ -219,6 +219,8 @@
  14: bytes in rate
  15: bytes out rate
  16: bytes out rate
+ 17: gpc1
+ 18: gpc1 rate
 
 d) Table Switch Message