sh: Add cpu-features header to asm/Kbuild.

This is used by the libc for parsing CPU capability flags passed
via the ELF auxvt, needed for run-time selection of atomic opcodes
amongst other things.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

include/asm-sh/Kbuild

 include include/asm-generic/Kbuild.asm
+
+header-y += cpu-features.h