add parenthesis around expression

Add parenthesis around expression before casting to boolean. This fixes:

main-window.vala:202.8-202.57: error: Condition must be boolean if ((bool) event.changed_mask & Gdk.WindowState.STICKY && this.visible) { ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ main-window.vala:204.9-204.73: error: Equality operation: bool' and Gdk.WindowState' are incompatible if (((bool) event.new_window_state & Gdk.WindowState.STICKY) == false) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Signed-off-by: Christian Hesse mail@eworm.de