tests: add missing // no-system-llvm annotation to rust-lang#69841 test.

Author: eddyb

src/test/ui/issues/issue-69841.rs

@@ -2,6 +2,7 @@
 // LLVM bug which needed a fix to be backported.
 
 // run-pass
+// no-system-llvm
 
 fn main() {
     let buffer = [49u8, 10];