include: Fix __uuidof(IUnknown) and IID_IUnknown in MinGW C++.
Before:
#define INITGUID
#include <unknwn.h>
const GUID& a() { return __uuidof(IUnknown); }
const GUID& b() { return IID_IUnknown; }
$ clang -xc++ -target i386-pc-windows-gnu -nostdinc -I include -I include/msvcrt a.cpp -masm=intel -nostdlib
a.cpp:3:26: warning: instantiation of variable '__wine_uuidof<IUnknown>::uuid' required here, but no definition is available [-Wundefined-var-template]
[...]
1 warning generated.
/usr/bin/ld: /tmp/a-7e5d2f.o:a.cpp:(.rdata$.refptr._IID_IUnknown+0x0): undefined reference to `IID_IUnknown'
/usr/bin/ld: /tmp/a-7e5d2f.o:a.cpp:(.rdata$.refptr.__ZN13__wine_uuidofI8IUnknownE4uuidE+0x0): undefined reference to `__wine_uuidof<IUnknown>::uuid'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
After: Creates an a.exe with no warnings. (With a() as entry point; figuring out how to give it a real entry point is offtopic to this MR.)