Add default suppression specification for webkitgtk
    
    This patch adds default suppression specifications for webkitgtk and
    webkit2gtk so that unnecessary ABI artifacts are dropped from the
    in-memory IR.  This dramatically helps to lower the memory usage of
    libabigail while analyzing the webkit2gtk libraries.
    
    	* default.abignore: New suppression specifications for webkitgtk.
    
    Signed-off-by: Dodji Seketeli <dodji@redhat.com>