Correct the bug of require WebKit name for GTK3 backend
Correct the bug of require WebKit name for GTK3 backend

The package WebKit is named as WebKit2 in Ubuntu distribution.
pllim/ginga,ejeschke/ginga,naojsoft/ginga,ejeschke/ginga,ejeschke/ginga,pllim/ginga,naojsoft/ginga,naojsoft/ginga,pllim/ginga