[gdb/testsuite] Fix full buffer in gdb.rust/dwindex.exp

[gdb/testsuite] Fix full buffer in gdb.rust/dwindex.exp

On ubuntu 18.04.5, I run into:
...
(gdb) mt print objfiles dwindex^M
^M
Object file build/gdb/testsuite/outputs/gdb.rust/dwindex/dwindex:  \
  Objfile at 0x55dab0b87a50, bfd at 0x55dab0b0cfa0, 1095 minsyms^M
^M
Psymtabs:^M
vendor/compiler_builtins/src/int/specialized_div_rem/mod.rs at 0x55dab0db0720^M
  ...
library/std/src/sys/unix/stdio.rs at 0x55dab0d96320^M
ERROR: internal buffer is full.
UNRESOLVED: gdb.rust/dwindex.exp: check if index present
...

Fix this by using -lbl in proc ensure_gdb_index.

Tested on x86_64-linux.