Bump libtool version (not package version) to 4:0:0.

Bump libtool version (not package version) to 4:0:0.

The last release (0.4.0) had the libtool version (current:revision:age)
set to 3:0:0. Since this release adds, removes, and changes interfaces,
the new version is 4:0:0.

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

This changes the library filename (e.g. on Linux) from libsigrok.so.3.0.0
to libsigrok.so.4.0.0, the SONAME (+symlink) becomes libsigrok.so.4.