tst-setcontext10.c: Undef _FORTIFY_SOURCE

When _FORTIFY_SOURCE is defined to 2, ____longjmp_chk is called,
instead of longjmp.  ____longjmp_chk compares the relative stack
values to decide if it is called from a stack frame which called
setjmp.  If not, ____longjmp_chk assumes that an alternate signal
stack is used.  Since comparing the relative stack values isn't
reliable with user context, when there is no signal, ____longjmp_chk
will fail.  Undefine _FORTIFY_SOURCE to avoid ____longjmp_chk in
user context test.