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

Makefile

@@ -70,7 +70,7 @@ $(filter %-softmmu,$(SUBDIR_RULES)): libqemu_common.a
 
 $(filter %-user,$(SUBDIR_RULES)): libuser.a
 
-libuser.a:
+libuser.a: $(GENERATED_HEADERS)
 	$(call quiet-command,$(MAKE) $(SUBDIR_MAKEFLAGS) -C libuser V="$(V)" TARGET_DIR="libuser/" all,)
 
 ROMSUBDIR_RULES=$(patsubst %,romsubdir-%, $(ROMS))