build: Use explicit config phony

Style unification.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/Makefile.config

@@ -48,4 +48,4 @@ endef
 $(eval $(config-header-rule))
 
 config: $(CONFIG_HEADER)
-PHONY += config
+.PHONY: config