[IA64] delete offsets.h in mrproper, not in clean rule

Patch from Tigran Aivazian:
On IA64 architecture the "make clean" does not correspond to its
definition:

# make clean     Delete most generated files
#                Leave enough to build external modules

because it deletes include/asm-ia64/offsets.h file which can easily be
included by a module, e.g. via this header path:

In file included from include/linux/thread_info.h:21,
from include/linux/spinlock.h:12,
from include/linux/capability.h:45,
from include/linux/sched.h:7,
from include/asm/uaccess.h:37,

So the fix is to remove this file from CLEAN_FILES and add it to
MRPROPER_FILES, so that it is NOT removed by "make clean" and removed by
"make mrproper".
Signed-off-by: Tony Luck <tony.luck@intel.com>

arch/ia64/Makefile

@@ -82,7 +82,9 @@ unwcheck: vmlinux
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)
 
-CLEAN_FILES += include/asm-ia64/.offsets.h.stamp include/asm-ia64/offsets.h vmlinux.gz bootloader
+CLEAN_FILES += include/asm-ia64/.offsets.h.stamp vmlinux.gz bootloader
+
+MRPROPER_FILES += include/asm-ia64/offsets.h
 
 prepare: include/asm-ia64/offsets.h