make: clean up obj-ext-src-y objects on "make clean"

This is needed for lib/rpc.pb-c.{d,o} to be removed by "make clean".
Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

scripts/Makefile.build

@@ -110,6 +110,7 @@ $(foreach file,						\
 			$(addprefix $(obj)/,$(notdir $(file:.o=))))))
 all-objs += $(__obj-ext-src-y)
 deps += $(__obj-ext-src-y:.o=.d)
+cleanup-y += $(__obj-ext-src-y) $(__obj-ext-src-y:.o=.d)
 endif
 
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