SUSv4 merely claims that ucontext_t is defined as a structure type (in
<signal.h>), and at least /usr/include/sys/ucontext.h in
glibc-headers-2.26-15.fc27.x86_64 now defines it as

  typedef struct ucontext_t { ... } ucontext_t;

instead of as

  typedef struct ucontext { ... } ucontext_t;

as was done in older glibc, so referring to it as "struct ucontext" (C/C++) or
"ucontext" (C++) no longer works.

Change-Id: I3d515a4ce7e5a49d0277136cad2b982bdf799ac5