Fix declaration of lock_set_foreground_window for pinnetry-w32.

* w32/main.c (lock_set_foreground_window): Remove WINUSERAPI
qualifier.

WINUSERAPI is expanded to dllimport which is not what we want if we
load the symbol at runtime. This produces an error with mingw-w64
4.6.3-14+8

(backport from a gpg4win patch)