Fix: add subdir-objects to Makefile.am

Fix: add subdir-objects to Makefile.am

Fixes the following warning introduced by the runas worker changes. Use
the same technique used in src/bin/lttng/Makefile.am.

src/common/Makefile.am:17: warning: source file 'sessiond-comm/unix.c' is in a subdirectory,
src/common/Makefile.am:17: but option 'subdir-objects' is disabled
automake: warning: possible forward-incompatibility.
automake: At least a source file is in a subdirectory, but the 'subdir-objects'
automake: automake option hasn't been enabled.  For now, the corresponding output
automake: object file(s) will be placed in the top-level directory.  However,
automake: this behaviour will change in future Automake versions: they will
automake: unconditionally cause object files to be placed in the same subdirectory
automake: of the corresponding sources.
automake: You are advised to start using 'subdir-objects' option throughout your
automake: project, to avoid future incompatibilities.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>