UAPI: Make arch/sparc/include/uapi/asm/sigcontext.h non-empty

arch/sparc/include/uapi/asm/sigcontext.h was emitted by the UAPI disintegration
script as an empty file because the parent file had no UAPI stuff in it,
despite being marked with "header-y".

Unfortunately, the patch program deletes resultant empty files when applying a
kernel patch.

So just stick a comment in there as a placeholder.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: David S. Miller <davem@davemloft.net>
cc: sparclinux@vger.kernel.org