Add extern reference to __libc_sigaction in sigaction.c for AVR32 arch

Signed-off-by: Hans-Christian Egtvedt <hans-christian.egtvedt@atmel.com>

libc/sysdeps/linux/avr32/sigaction.c

@@ -16,6 +16,8 @@ extern void __default_rt_sa_restorer(void);
 
 /* Experimentally off - libc_hidden_proto(memcpy) */
 
+extern __typeof(sigaction) __libc_sigaction;
+
 /*
  * If act is not NULL, change the action for sig to *act.
  * If oact is not NULL, put the old action for sig in *oact.