gcc-7/i386: markup intentional fallthroughs

Summary:
Markup i386 code paths resolving compilation
failure under i386 with gcc-7.

Signed-off-by: James Page <james.page@ubuntu.com>
Closes https://github.com/facebook/rocksdb/pull/2700

Differential Revision: D5583047

Pulled By: maysamyabandeh

fbshipit-source-id: fe31bcfeaf7cd2d3f51b55f5ae0b3b0cb3788fbc

util/murmurhash.cc

@@ -113,8 +113,8 @@ unsigned int MurmurHash2 ( const void * key, int len, unsigned int seed )
 
     switch(len)
     {
-    case 3: h ^= data[2] << 16;
-    case 2: h ^= data[1] << 8;
+    case 3: h ^= data[2] << 16; // fallthrough
+    case 2: h ^= data[1] << 8; // fallthrough
     case 1: h ^= data[0];
         h *= m;
     };