[SPARC64]: Fix make headers_install

A minor typo in the include/asm-sparc64/Kbuild file prevents the make
headers_install from building a useful tree of kernel headers for
sparc64.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Tom "spot" Callaway <tcallawa@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>