Fix fallthrough warnings in murmurhash

Fix two -Wimplicit-fallthrough warnings in the murmurhash function:

  ../src/murmurhash.c: In function 'murmurhash':
  ../src/murmurhash.c:71:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
     71 |     case 3: k ^= (tail[2] << 16);
        |             ~~^~~~~~~~~~~~~~~~~~
  ../src/murmurhash.c:72:5: note: here
     72 |     case 2: k ^= (tail[1] << 8);
        |     ^~~~
  ../src/murmurhash.c:72:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
     72 |     case 2: k ^= (tail[1] << 8);
        |             ~~^~~~~~~~~~~~~~~~~
  ../src/murmurhash.c:74:5: note: here
     74 |     case 1:
        |     ^~~~

src/murmurhash.c

@@ -69,8 +69,9 @@ murmurhash (const char *key, uint32_t len, uint32_t seed) {
   // remainder
   switch (len & 3) { // `len % 4'
     case 3: k ^= (tail[2] << 16);
+      // fallthrough
     case 2: k ^= (tail[1] << 8);
-
+      // fallthrough
     case 1:
       k ^= tail[0];
       k *= c1;