Update dll_symbol.h (#5781)

- fix compile switch
- closes https://github.com/assimp/assimp/issues/5777

contrib/poly2tri/poly2tri/common/dll_symbol.h

@@ -31,7 +31,7 @@
 
 #pragma once
 
-#if defined(_WIN32)
+#if defined(_MSC_VER)
 #  pragma warning( disable: 4273)
 #  define P2T_COMPILER_DLLEXPORT __declspec(dllexport)
 #  define P2T_COMPILER_DLLIMPORT __declspec(dllimport)