vscode: export MI_XSD_PATH and LD_LIBRARY_PATH in gdb wrapper

vscode: export MI_XSD_PATH and LD_LIBRARY_PATH in gdb wrapper

vscode fails to launch a session daemon using the libtool wrapper on machines
that don't have lttng installed since the XSD can't be loaded.

Export the MI_XSD_PATH to point to the file in the source tree to allow the
daemon to launch.

Moreover, launching the session daemon fails since it can't find
liblttng-ctl.so.0. Adding the library's .libs to LD_LIBRARY_PATH works around
the issue.

Change-Id: I52929b858917e5a103e067e6861cbdb5172744be
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>