[gdb/testsuite] Fix gdb.dwarf2/enqueued-cu-base-addr.exp with cc-with-gnu-debuglink

When running test-case gdb.dwarf2/enqueued-cu-base-addr.exp with target board
cc-with-gnu-debuglink, I run into:
...
(gdb) PASS: gdb.dwarf2/enqueued-cu-base-addr.exp: ptype foo
maint print symbols -objfile enqueued-cu-base-addr^M
(gdb) FAIL: gdb.dwarf2/enqueued-cu-base-addr.exp: CU addr found
...

The problem is that the CU we're trying to print is in objfile
enqueued-cu-base-addr.debug instead of enqueued-cu-base-addr.

Fix this by replacing "-objfile enqueued-cu-base-addr" with "-source cu2".

Tested on x86_64-linux.