build-sys: include .version in EXTRA_DIST

build-sys: include .version in EXTRA_DIST

.version is created in the top_srdir which when building normally
doesn't present a problem. When make distcheck is run, the source
directory is made read-only and it fails with permission denied.

The version of misc/git-version-gen is old and is missing the
instruction to add .version to EXTRA_DIST. This commit does just
that.

make distcheck now passes.

References:
 https://github.com/gagern/gnulib/blob/master/build-aux/git-version-gen
 https://lists.gnu.org/archive/html/bug-gnulib/2011-08/msg00000.html

Signed-off-by: Craig Small <csmall@enc.com.au>