Add bihash statistics hook

Example / unit-test in .../src/plugins/unittest/bihash_test.c

Change-Id: I23fd0ba742d65291667a755965aee1a3d3477ca2
Signed-off-by: Dave Barach <dave@barachs.net>
diff --git a/src/vppinfra/bihash_40_8.h b/src/vppinfra/bihash_40_8.h
index b85adb0..b50e5eb 100644
--- a/src/vppinfra/bihash_40_8.h
+++ b/src/vppinfra/bihash_40_8.h
@@ -15,6 +15,8 @@
 
 #undef BIHASH_TYPE
 #undef BIHASH_KVP_PER_PAGE
+#undef BIHASH_32_64_SVM
+#undef BIHASH_ENABLE_STATS
 
 #define BIHASH_TYPE _40_8
 #define BIHASH_KVP_PER_PAGE 4