Fix cleaning in scripts/mod

Make sure devicetable-offsets.h is cleaned in the scripts/mod directory
Signed-off-by: Andreas Schwab <schwab@linux-m68k.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/mod/Makefile

@@ -37,6 +37,8 @@ scripts/mod/devicetable-offsets.s: scripts/mod/devicetable-offsets.c FORCE
 $(obj)/$(devicetable-offsets-file): scripts/mod/devicetable-offsets.s
 	$(call cmd,offsets)
 
+targets += $(devicetable-offsets-file)
+
 # dependencies on generated files need to be listed explicitly
 
 $(obj)/modpost.o $(obj)/file2alias.o $(obj)/sumversion.o: $(obj)/elfconfig.h