Fix build errors in mingw

Cryptic linking errors would appear when trying to compile with mingw

The errors mentioned __mingw_uuidof<ID3D12Heap>,
__mingw_uuidof<ID3D12Device8> and __mingw_uuidof<ID3D12Device10> as
undefined references.

Author: darksylinc

src/D3D12MemAlloc.cpp

@@ -33,6 +33,12 @@
     #include <shared_mutex>
 #endif
 
+#if !defined(_MSC_VER)
+#include <guiddef.h>
+// guiddef.h must be included first.
+#include <dxguids.h>
+#endif
+
 ////////////////////////////////////////////////////////////////////////////////
 ////////////////////////////////////////////////////////////////////////////////
 //