Put the libbabeltrace-ctf specific parts of babeltrace.pc into a babeltrace-ctf.pc...

Put the libbabeltrace-ctf specific parts of babeltrace.pc into a babeltrace-ctf.pc file

On some distributions libbabeltrace and libbabeltrace-ctf are packaged
into separate packages. This means they should have their own pkg-config
file.

The babeltrace-ctf.pc file requires the babeltrace.pc one.

With this change, to have the ctf support in libbabeltrace a pkg-config
user will need to specify the babeltrace-ctf package. To compile and
link with only the base libbabeltrace, the package babeltrace would be
used.

Fixes #550

Signed-off-by: Yannick Brosseau <yannick.brosseau@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>