LayoutLine: Remove unnecessary forward declaration
LayoutLine: Remove unnecessary forward declaration

The forward declaration of Glib::wrap(PangoLayoutLine*, bool) became
unnecessary when the LayoutLineTraits struct was moved to layout.ccg
in commit 57d8bceba336e972a4967db6872a373eac1833b9.

GNOME/pangomm,GNOME/pangomm,GNOME/pangomm,GNOME/pangomm