Add fallthrough comment to silence gcc-7 warning

Until we switch to C++17 and can use the proper attributes there.
Fixes #1437

src/hash.cc

@@ -50,8 +50,8 @@ size_t hash_data(const char* input, size_t len)
     uint32_t key = 0;
     switch (len & 3)
     {
-        case 3: key ^= tail[2] << 16;
-        case 2: key ^= tail[1] << 8;
+        case 3: key ^= tail[2] << 16; /* fallthrough */
+        case 2: key ^= tail[1] << 8;  /* fallthrough */
         case 1: key ^= tail[0];
                 key *= c1;
                 key = rotl(key,15);