sys.stringtorune doesn't need a length parameter.

R=rsc
DELTA=7  (0 added, 0 deleted, 7 changed)
OCL=16600
CL=16630
diff --git a/src/runtime/runtime.h b/src/runtime/runtime.h
index f590472..f182aeb 100644
--- a/src/runtime/runtime.h
+++ b/src/runtime/runtime.h
@@ -293,4 +293,4 @@
  */
 void	sys·readfile(string, string, bool);
 void	sys·bytestorune(byte*, int32, int32, int32, int32);
-void	sys·stringtorune(string, int32, int32, int32, int32);
+void	sys·stringtorune(string, int32, int32, int32);