signal.c: Don't mark ruby_abort as NORETURN on Solaris

to suppress a warning.

```
signal.c: In function 'ruby_abort':
signal.c:987:1: warning: 'noreturn' function does return
 }
 ^
```

1 files changed, 2 insertions, 0 deletions

diff --git a/signal.c b/signal.c
index c33c756948..3c31935fbc 100644
--- a/signal.c
+++ b/signal.c
@@ -970,7 +970,9 @@ sigbus(int sig SIGINFO_ARG)
 }
 #endif
 
+#ifndef __sun
 NORETURN(static void ruby_abort(void));
+#endif
 
 static void
 ruby_abort(void)