crt: Update the decoration of the EditWndProc symbol to EditWndProc@20

This function seems to be undocumented (we don't have any declaration
of it in any header), but this is the form it gets dumped from current
user32.dll.

Signed-off-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Jonathan Yong <10walls@gmail.com>