package/pkg-generic.mk: remove double colon in .stamp_built

This issue was introduced back in 2009 with commit
e11fe847b2f545446fc3300dd2ba88fd5da05756 that created the generic
package infrastructure.

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>
(cherry picked from commit bd0d5498ea00c29b22bb8c87b8f593b356deb469)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-generic.mk

@@ -276,7 +276,7 @@ $(BUILD_DIR)/%/.stamp_configured:
 	$(Q)touch $@
 
 # Build
-$(BUILD_DIR)/%/.stamp_built::
+$(BUILD_DIR)/%/.stamp_built:
 	@$(call step_start,build)
 	@$(call MESSAGE,"Building")
 	$(foreach hook,$($(PKG)_PRE_BUILD_HOOKS),$(call $(hook))$(sep))