configure.ac: Bump libtool/library version from 5:0:1 to 6:0:2.

The libtool current:revision:age numbers change from 5:0:1 to 6:0:2
since new APIs were added (but no existing ones were changed or
removed). Thus, revision is set to 0, current and age are bumped.

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.4.1.0 to libsigrokdecode.so.4.2.0, the SONAME
(+symlink) remains the same, though (libsigrokdecode.so.4).