xtensa: fix build failure in xtensa/kernel/signal.c

Caused by commit 3785006ac3c8941feb63097c416de92114a6bc39

    "xtensa: don't mask signals if we fail to setup signal stack"

It assigns a return value to "ret", but there is no such variable
anywhere in scope.  Create one.

Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Chris Zankel <chris@zankel.net>
Acked-by: Matt Fleming <matt.fleming@intel.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>