powerpc: Remove non-uapi linkage.h export

powerpc: Remove non-uapi linkage.h export

The arch/powerpc/include/asm/linkage.h is being unintentionally exported
in the kernel headers since commit e65746272fae (consolidate
cond_syscall and SYSCALL_ALIAS declarations) when
arch/powerpc/include/uapi/asm/linkage.h was deleted but the header-y not
removed from the Kbuild file. This happens because Makefile.headersinst
still checks the old asm/ directory if the specified header doesn't
exist in the uapi directory.

The asm/linkage.h shouldn't ever have been exported anyway. No other
arch does and it doesn't contain anything useful to userland, so remove
the header-y line from the Kbuild file which triggers the export.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>