Forward-compatible fix for ucontext_t

The Android C library <signal.h> will soon be modified to declare
ucontext_t in <signal.h>. To avoid conflicts when this happens, only
define the type when needed.

See b/issue?id=2189142

Change-Id: I297b8e6f9fba8fb99c5f799b0d7e7f4dbda914bb
1 file changed