Don't do pointer arithmetics on void*

For instance MSVC doesn't like that.

Signed-off-by: Tor Lillqvist <tml@iki.fi>

gobject/gatomicarray.c

@@ -77,7 +77,7 @@ freelist_alloc (gsize size, gboolean reuse)
 
   real_size = sizeof (gsize) + MAX (size, sizeof (FreeListNode));
   mem = g_slice_alloc (real_size);
-  mem += sizeof (gsize);
+  mem = ((char *) mem) + sizeof (gsize);
   G_ATOMIC_ARRAY_DATA_SIZE (mem) = size;
   return mem;
 }