Fix: Improper use of LDFLAGS in libbabeltrace-ctf's Makefile.am

*_LDFLAGS should not be used to link libraries since automake won't identify
them as dependencies of the target. This would cause the target to not be
rebuilt when one of the CTF static library was modified.

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