gdb: lx-symbols: do not error out on monolithic build

This avoids spurious message:
(gdb) lx-symbols
loading vmlinux
No source file named kernel/module/main.c.

Link: https://lkml.kernel.org/r/Zy5ALByQtpO-ddh4@Z926fQmE5jqhFMgp6


Signed-off-by: Etienne Buira <etienne.buira@free.fr>
Cc: Andrew Ballance <andrewjballance@gmail.com>
Cc: Kieran Bingham <kbingham@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>