fix compilation on BigEndian

cannot pass a function by reference where an lvalue is expected
(only applies to bigendian, where a macro expands to a byteswap function)

Closes https://github.com/assimp/assimp/issues/613

code/Bitmap.cpp

@@ -84,7 +84,12 @@ namespace Assimp {
 
     template<typename T>
     inline std::size_t Copy(uint8_t* data, T& field) {
+#ifdef AI_BUILD_BIG_ENDIAN
+        T field_swapped=AI_BE(field);
+        std::memcpy(data, &field_swapped, sizeof(field)); return sizeof(field);
+#else
         std::memcpy(data, &AI_BE(field), sizeof(field)); return sizeof(field);
+#endif
     }
 
     void Bitmap::WriteHeader(Header& header, IOStream* file) {