mingw-w64 defines __forceinline (and therefore FORCEINLINE) as
"extern __inline__ __attribute__((__always_inline__,__gnu_inline__)). This means
that COM inline wrappers specify multiple storage classes and hence cannot be
compiled.

Wine defines FORCEINLINE simply as "inline" (and uses "static" everywhere), so
this is a non-issue for Wine. However, since Wine and mingw-w64 share the source
code of widl and of most IDL headers, this patch changes the definition for both
projects.

There's no reason to force inlining here, especially since the wrappers need to
be manually enabled, and we don't need to match PSDK semantics where these
wrappers don't even exist.

In practice, use "__inline__" instead of "inline" for GNU C targets, to preserve
compatibility with C89 in mingw-w64 headers.