uclibc: define struct ucontext for or1k

Add missing define for struct ucontext for OpenRISC.

Fixes:
  http://autobuild.buildroot.net/results/d8715a2b3783d877edf91bb14a80d36eac749b36



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>