add fixme to improve error matching

Author: jdonszelmann

tests/ui/macros/borrowck-error-in-macro.rs

@@ -1,5 +1,6 @@
 //@ aux-build: borrowck-error-in-macro.rs
 //@ error-pattern: a call in this macro requires a mutable binding due to mutable borrow of `d`
+//FIXME: remove error-pattern (see #141896)
 
 extern crate borrowck_error_in_macro as a;