unicore32: Fix build error

unicore32 builds fail with

  arch/unicore32/kernel/signal.c: In function ‘setup_frame’:
  arch/unicore32/kernel/signal.c:257: error: ‘usig’ undeclared (first use in this function)
  arch/unicore32/kernel/signal.c:279: error: ‘usig’ undeclared (first use in this function)
  arch/unicore32/kernel/signal.c: In function ‘handle_signal’:
  arch/unicore32/kernel/signal.c:306: warning: unused variable ‘tsk’
  arch/unicore32/kernel/signal.c: In function ‘do_signal’:
  arch/unicore32/kernel/signal.c:376: error: implicit declaration of function ‘get_signsl’
  make[1]: *** [arch/unicore32/kernel/signal.o] Error 1
  make: *** [arch/unicore32/kernel/signal.o] Error 2

Bisect points to commit 649671c9 ("unicore32: Use get_signal()
signal_setup_done()").

This code never even compiled.  Reverting the patch does not work, since
previously used functions no longer exist, so try to fix it up.  Compile
tested only.

Fixes: 649671c9 ("unicore32: Use get_signal() signal_setup_done()")
Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>