consistency: use struct __ucontext instead of ucontext_t in prototypes

this is necessary to avoid build errors if feature test macros are not
properly defined when including ucontext.h

include/ucontext.h

@@ -14,7 +14,7 @@ struct __ucontext;
 
 int  getcontext(struct __ucontext *);
 void makecontext(struct __ucontext *, void (*)(void), int, ...);
-int  setcontext(const ucontext_t *);
+int  setcontext(const struct __ucontext *);
 int  swapcontext(struct __ucontext *, const struct __ucontext *);
 
 #ifdef __cplusplus