UAPI: The tile arch uses the generic ucontext.h file

Move the header-y and generic-y lines for ucontext.h from
arch/tile/include/asm/Kbuild to the uapi/ Kbuild as the asm-generic variant is
used.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Chris Metcalf <cmetcalf@tilera.com>