configure.ac: Bump libtool/library version from 6:0:2 to 6:1:2.

configure.ac: Bump libtool/library version from 6:0:2 to 6:1:2.

The libtool current:revision:age numbers change from 6:0:2 to 6:1:2 since
no new APIs were added, changed or removed (but the source code did change).

Details:
http://www.gnu.org/software/libtool/manual/libtool.html#Updating-version-info

This changes the library filename (e.g. on Linux) from
libsigrokdecode.so.4.2.0 to libsigrokdecode.so.4.2.1, the SONAME
(+symlink) remains the same, though (libsigrokdecode.so.4).