Merge pull request #418 from manas-kulkarni/master

Make Vulkan include optional
diff --git a/include/vk_mem_alloc.h b/include/vk_mem_alloc.h
index 2307325..408895f 100644
--- a/include/vk_mem_alloc.h
+++ b/include/vk_mem_alloc.h
@@ -127,7 +127,9 @@
 extern "C" {
 #endif
 
+#if !defined(VULKAN_H_)
 #include <vulkan/vulkan.h>
+#endif
 
 #if !defined(VMA_VULKAN_VERSION)
     #if defined(VK_VERSION_1_3)