Bump libtool version (not package version) to 3:0:0.

The last release (0.3.0) had the libtool version (current:revision:age)
set to 2:0:0. Since this release removes and changes interfaces, the new
version is 3:0:0.

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

This changes the library filename (e.g. on Linux) from
libsigrokdecode.so.2.0.0 to libsigrokdecode.so.3.0.0, the SONAME
(+symlink) becomes libsigrokdecode.so.3.