widget: Do not mix function linkage
widget: Do not mix function linkage

This prevents a possible crash on windows 32 bit.
The linkage of UnhookWindowsHookEx is WINAPI which is __stdcall while
callback for g_clear_pointer is C. This could cause stack pointer
corruption depending on compiler flags.
On __stdcall linkage function change the stack pointer while returning
from a function removing the parameters. On C linkage function leave
the stack pointer unchanged. So if the compiler call a __stdcall
function as a C function it expect the stack pointer to be unchanged
causing the pointer to be inconsistent by an offset.

Signed-off-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>

fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk