Allow compiling the Syncthing library interface with MSVC

I haven't tested linking and execution yet; it would probably require to
use e.g. `mingw-w64-ucrt-x86_64-go` from MSYS2 (instead of
`mingw-w64-x86_64-go` where the stdlib clashes).

syncthing/interface.cpp

@@ -10,6 +10,11 @@
 #define __declspec(foo)
 #endif
 
+// enforce use of the C header for complex types as cgo's typedefs rely on it
+#ifdef _MSC_VER
+#define _CRT_USE_C_COMPLEX_H
+#endif
+
 #include "libsyncthinginternal.h"
 
 #include <atomic>