Refactored VmaBlockVector::SortByFreeSize to fix compilation on XCode 13.4

Fixes #269. Thanks @richard-lunarg !
diff --git a/include/vk_mem_alloc.h b/include/vk_mem_alloc.h
index 9a639c1..81b03b6 100644
--- a/include/vk_mem_alloc.h
+++ b/include/vk_mem_alloc.h
@@ -12732,7 +12732,7 @@
 void VmaBlockVector::SortByFreeSize()

 {

     VMA_SORT(m_Blocks.begin(), m_Blocks.end(),

-        [](auto* b1, auto* b2)

+        [](VmaDeviceMemoryBlock* b1, VmaDeviceMemoryBlock* b2) -> bool

         {

             return b1->m_pMetadata->GetSumFreeSize() < b2->m_pMetadata->GetSumFreeSize();

         });