Add missed chunkGC timings start

Pointed out by Andi in Spigot #53

CraftBukkit-Patches/0010-Spigot-Timings.patch

@@ -563,6 +563,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
          this.ao();
 +        timings.doSounds.stopTiming(); // Spigot
  
++        timings.doChunkGC.startTiming();// Spigot
          this.getWorld().processChunkGC(); // CraftBukkit
 +        timings.doChunkGC.stopTiming(); // Spigot
      }