Both flashing a window and setting the window opacity were using
incorrect declarations for function pointers. They were missing the
WINAPI annotation as defined in windows.h. As a result, the stack
could be corrupted when these functions were invoked.
Fixes https://bugzilla.gnome.org/show_bug.cgi?id=689235
(cherry picked from commit 5637ef1f97ee46666c97707ed7f6bae459007163)