Fix missing files in distribution target

When running autoreconf -f -i on a distributed tarball, the configuration fails with the following error:
	autoreconf -f -i
sh: 1: ./get-version.sh: not found
configure.ac:7: error: AC_INIT should be called with package and version arguments
/usr/share/aclocal-1.14/init.m4:29: AM_INIT_AUTOMAKE is expanded from...
configure.ac:7: the top level

Files VERSION and get-version.sh are required to run autoreconf, on the
sources from a 'make dist' tarball.
Add them to the EXTRA_DIST target.