gdk/gtk: Fix the build for GTK+ < 2.21.7

GDK_KEY_ got introduced in 2.21.8 but we still intend to support
RHEL6 and Debian6.0. Use the old version of the symbols.

2013-08-18  Gwenael Casaccio  <gwenael.casaccio@gmail.com>

	* mk_enums.awk: Use the old names for GDK key symbols.