Fix a make -j race

Make libuser.a depend on $(GENERATED_HEADERS) too so make -j won't start
building it before the headers exist.  (There may be more bugs like this
but at least this makes (g)make -j4 started from scratch on a quadcore
now always complete here again.)

Signed-off-by: Juergen Lock <nox@jelal.kn-bremen.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
