src/libmain/stack.cc: fix 'ucontext' usage on glibc-2.26

Build fails as:

$ make
  CXX    src/libmain/stack.o
src/libmain/stack.cc: In function 'void nix::sigsegvHandler(int, siginfo_t*, void*)':
src/libmain/stack.cc:21:21: error: 'ucontext' was not declared in this scope
     sp = (char *) ((ucontext *) ctx)->uc_mcontext.gregs[REG_RSP];
                     ^~~~~~~~
src/libmain/stack.cc:21:21: note: suggested alternative: 'ucontext_t'
     sp = (char *) ((ucontext *) ctx)->uc_mcontext.gregs[REG_RSP];
                     ^~~~~~~~
                     ucontext_t

It's caused by upstream rename:
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=251287734e89a52da3db682a8241eb6bccc050c9



which basically changes
    typedef struct ucontext {} ucontext_t;
to
    typedef struct ucontext_t {} ucontext_t;

The change uses ucontext_t.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>