git-gui: set version 0.21

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

GIT-VERSION-GEN

diff --git a/GIT-VERSION-GEN b/GIT-VERSION-GEN
index a88b682..92373d2 100755
--- a/GIT-VERSION-GEN
+++ b/GIT-VERSION-GEN
@@ -1,7 +1,7 @@
 #!/bin/sh
 
 GVF=GIT-VERSION-FILE
-DEF_VER=0.20.GITGUI
+DEF_VER=0.21.GITGUI
 
 LF='
 '