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

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

The libtool current:revision:age numbers change from 1:1:0 to 1:2:0
(i.e., revision is increased) since the library source code has changed,
but no interfaces were added or changed or removed.

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

This changes the library filename (e.g. on Linux) from libsigrok.so.1.0.1
to libsigrok.so.1.0.2, but the SONAME (+symlink) remains the same
(libsigrok.so.1) since this release is API- and ABI-compatible with the last.