Bump version to 2.1.0~beta2

I forgot to bump the configure.ac version before tagging the 2.1.0~beta1
release. Since we cannot remove old tags (see “On Re-tagging” in git-tag(1)),
we have to call this release 2.1.0~beta2.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>

configure.ac

@@ -2,7 +2,7 @@
 m4_define([gnt_version_major], [2])
 m4_define([gnt_version_minor], [1])
 m4_define([gnt_version_revision], [0])
-m4_define([gnt_version_suffix], [~beta0])
+m4_define([gnt_version_suffix], [~beta2])
 m4_define([gnt_version_full],
           m4_format([%d.%d.%d%s],
                     gnt_version_major, gnt_version_minor,