nios2: Remove ucontext.h from exported arch headers

Commit 92d5dd8c ("nios2: update pt_regs") removed the nios2 specific
ucontext.h, replacing it with the version from asm-generic. Thus it's no
longer necessary to include ucontext.h in exported headers.

Cc: Chung-Ling Tang <cltang@codesourcery.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <lftan@altera.com>