util: fix container_of() macro

util: fix container_of() macro

Fixes a bunch of warnings of the kind
../src/evdev.h:378:32: warning: variable 'f' is uninitialized when used here [-Wuninitialized]
        return container_of(dispatch, f, base);

Just typecasting NULL means we can ignore sample but for the type.

https://bugs.freedesktop.org/show_bug.cgi?id=100976

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>
Tested-by: Armin Krezović <krezovic.armin@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>