qga/commands-win32: Fix problem with redundant protype declaration

qga/commands-win32: Fix problem with redundant protype declaration

When compiling QEMU with MSYS2 on Windows, there is currently the
following error:

../qga/commands-win32.c:62:24: error: redundant redeclaration of
 'CM_Get_DevNode_PropertyW' [-Werror=redundant-decls]
   62 | CMAPI CONFIGRET WINAPI CM_Get_DevNode_PropertyW(
      |                        ^~~~~~~~~~~~~~~~~~~~~~~~
In file included from ../qga/commands-win32.c:26:
C:/tools/msys64/mingw64/x86_64-w64-mingw32/include/cfgmgr32.h:840:26: note:
 previous declaration of 'CM_Get_DevNode_PropertyW' was here
  840 |   CMAPI CONFIGRET WINAPI CM_Get_DevNode_PropertyW(DEVINST dnDevInst,
   const DEVPROPKEY *PropertyKey, DEVPROPTYPE *PropertyType, PBYTE PropertyBuffer,
   PULONG PropertyBufferSize, ULONG ulFlags);

Seems like this protype is sometimes available in the cfgmgr32.h
header, and sometimes not. Let's silence the compiler warning here
to let the build pass with -Werror, too.

Message-Id: <20200915114757.55635-1-thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>