x86/boot/build: Add cpustr.h to targets and remove clean-files

Files in $(targets) are always cleaned up. Move the 'targets' assignment
out of the ifdef and remove 'clean-files'.
Signed-off-by: NMasahiro Yamada <masahiroy@kernel.org>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200215063241.7437-1-masahiroy@kernel.org

arch/x86/boot/Makefile

@@ -57,11 +57,10 @@ $(obj)/cpu.o: $(obj)/cpustr.h
 
 quiet_cmd_cpustr = CPUSTR  $@
       cmd_cpustr = $(obj)/mkcpustr > $@
-targets += cpustr.h
 $(obj)/cpustr.h: $(obj)/mkcpustr FORCE
 	$(call if_changed,cpustr)
 endif
-clean-files += cpustr.h
+targets += cpustr.h
 
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