Make --allow-rs-prefix more reasonable.

Now slang only alarms when a function prefixed with 'rs' and was not declared
in one of the RS default headers (including rs_graphics.rsh). Therefore, you
don't need to (and shouldn't) pass this option to slang in general.
diff --git a/slang_rs.h b/slang_rs.h
index ad0c3da..0e07a59 100644
--- a/slang_rs.h
+++ b/slang_rs.h
@@ -25,6 +25,8 @@
 
 
  public:
+  static bool IsRSHeaderFile(const char *File);
+
   SlangRS(const char *Triple, const char *CPU, const char **Features);
 
   // The package name that's really applied will be filled in