maybe-uninitialized warning in Xtensa flush_window_regs()

As of QEMU v6.1.0:

C compiler for the host machine: gcc (gcc 10.3.1 "gcc (Alpine 10.3.1_git20210625) 10.3.1 20210625")

../linux-user/xtensa/signal.c: In function 'setup_rt_frame':
../linux-user/xtensa/signal.c:73:23: warning: 'osp' may be used uninitialized in this function [-Wmaybe-uninitialized]
   73 |         uint32_t ssp, osp;
      |                       ^~~