[gdb/testsuite] Fix gdb.dwarf2/gdb-add-index-symlink.exp

The test-case gdb.dwarf2/gdb-add-index-symlink.exp interpretes a failure to
add an index as a failure to add an index for a symlink:
...
if { [ensure_gdb_index $symlink] == -1 } {
    fail "Unable to call gdb-add-index with a symlink to a symfile"
    return -1
}
...

However, it's possible that the gdb-add-index also fails with a regular
file.  Add a check for that situation.

Tested on x86_64-linux.