vala: do not append a -dirty suffix to the valac version number

When vala is built inside a Git repository with uncommitted changes, the
valac version number has -dirty appended. This creates problems for
packages (e.g. Midori) which require a valac version number without
the -dirty suffix.

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>