Add -g that I should have included in PR #24932.

Note it is safe, with respect to autobuilds, to land before #24945.

(In other words, landing this sooner won't break things for anyone any
worse than they were already broken, since there are *other* tests
that also add `-g` to their flags via `compile-flags: -g`.)

src/test/run-pass/issue24687-embed-debuginfo.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 // aux-build:issue24687_lib.rs
+// compile-flags:-g
 
 extern crate issue24687_lib as d;