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

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

The libtool current:revision:age numbers change from 5:0:1 to 5:1:1
since no new APIs were added and no existing ones were changed or
removed. Thus, revision is incremented, current and age stay unchanged.

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.4.1.0 to libsigrok.so.4.1.1, the SONAME
(+symlink) remains the same, though (libsigrok.so.4).