qga: Fix compilation for old versions of MinGW

qga: Fix compilation for old versions of MinGW

While MinGW-w64 can compile the qga code, MinGW from Debian lenny
(gcc-mingw32 4.4.2-3) shows these errors:

In file included from qga/vss-win32.c:17:
qga/vss-win32/requester.h:31:
 error: expected »=«, »,«, »;«, »asm« or »__attribute__« before »requester_init«
qga/vss-win32/requester.h:32:
 error: expected »=«, »,«, »;«, »asm« or »__attribute__« before »requester_deinit«

The macro STDAPI is unknown, so add the missing include file which
defines it.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>