Fix: extra-version-git.sh redirect stderr to /dev/null

Running make in a git repo that does not contain any tag prints:

fatal: No names found, cannot describe anything.

in the make and make clean outputs.

It's fine to have no tag name available (extra-version-git.sh will
return the value 0), but we should not print an error in the make
output. Redirect this error to /dev/null.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Suggested-by: Michael Jeanson <mjeanson@efficios.com>