[gitg] Fix GSEAL with gtk 2.20

[Guilhem Bonnefille <guilhem bonnefille gmail com>]

Hi,

I tried to compile current gitg, but it failed:
implicit declaration of function ‘GTK_WIDGET_MAPPED’

Here is a patch for this.
-- 
Guilhem BONNEFILLE
-=- JID: guyou im apinc org MSN: guilhem_bonnefille hotmail com
-=- mailto:guilhem bonnefille gmail com
-=- http://nathguil.free.fr/

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