runtime: make go-nosys.c compile with !HAVE_SYSCALL

Change-Id: Ie35916bf73af07a2a6120fc8fbbd024b43e11a1d
Reviewed-on: https://go-review.googlesource.com/59910
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/libgo/runtime/go-nosys.c b/libgo/runtime/go-nosys.c
index 154f6b1..30222df 100644
--- a/libgo/runtime/go-nosys.c
+++ b/libgo/runtime/go-nosys.c
@@ -506,7 +506,7 @@
 
 #ifndef HAVE_SYSCALL
 int
-syscall(int number, ...)
+syscall(int number __attribute__ ((unused)), ...)
 {
   errno = ENOSYS;
   return -1;