Fixed building by Clang.

Fixed building by Clang.

src/qjs.c:347:19: error: variable 'signo' may be uninitialized when used
	here [-Werror,-Wconditional-uninitialized]
  347 |     if (kill(pid, signo) < 0) {
      |                   ^~~~~
src/qjs.c:294:31: note: initialize the variable 'signo' to silence this
	warning
  294 |     int                  signo, pid;
      |                               ^
      |                                = 0
1 error generated.