Blackfin: stop cleaning include/asm/asm-offsets.h

The file is no longer generated, so we don't want to clean it.
Reported-by: Vivi Li <vivi.li@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/blackfin/Makefile

@@ -133,7 +133,6 @@ KBUILD_CFLAGS += -Iarch/$(ARCH)/mach-$(MACHINE)/include
 KBUILD_CPPFLAGS	+= $(patsubst %,-I$(srctree)/%include,$(machdirs))
 
 CLEAN_FILES += \
-	arch/$(ARCH)/include/asm/asm-offsets.h \
 	arch/$(ARCH)/kernel/asm-offsets.s \
 
 archclean: