stubs: silence pedantic warning (NFC)

C requires that there is at least one declaration in a translation unit.
Because this file is ObjC and not ObjC++, this restriction applies.  Add
a declaration to silence the warning in the case that ObjC interop is
disabled.

Author: compnerd

stdlib/public/stubs/SwiftNativeNSXXXBaseARC.m

@@ -82,4 +82,8 @@ size_t swift_stdlib_NSStringHashValue(NSString *SWIFT_NS_RELEASES_ARGUMENT str,
   }
 }
 
+#else
+
+extern char ignore_pedantic_warning;
+
 #endif