remove macro definition of longjmp from setjmp.h

the C standard specifies that setjmp is a macro, but longjmp is a
normal function. a macro version of it would be permitted (albeit
useless) for C (not C++), but would have to be a function-like macro,
not an object-like one.

include/setjmp.h

@@ -33,7 +33,6 @@ int setjmp (jmp_buf);
 _Noreturn void longjmp (jmp_buf, int);
 
 #define setjmp setjmp
-#define longjmp longjmp
 
 #ifdef __cplusplus
 }