gdb: look for objects also in apps/ directory

For debugging OSv, we add to gdb a "osv syms" command which attempts
to load into gdb the same shared-objects that were loaded on OSv.

We looked for these objects in a bunch of places (build directory,
modules/ directory, etc.). This patch also adds to this path the apps/
directory (where stuff from osv-apps.git is compiled). This is
convenient when debugging OSv running various applications.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>