backport of https://github.com/apple/swift-corelibs-libdispatch/pull/559

Index: src/io.c
--- src/io.c.orig
+++ src/io.c
@@ -2308,7 +2308,7 @@ _dispatch_operation_advise(dispatch_operation_t op, si
 		default: (void)dispatch_assume_zero(err); break;
 	}
 #else
-#error "_dispatch_operation_advise not implemented on this platform"
+	(void)err;
 #endif // defined(F_RDADVISE)
 #endif // defined(_WIN32)
 }