compat: rename socket.h to socket.hpp

compat: rename socket.h to socket.hpp

`socket.h` is only included in C++ code, so make it a C++ file.  This
makes it possible for an upcoming patch to make it use `bt2c::Logger`.

Change-Id: I42c9fd08a343357c116cf85675f36997cc1f3f2e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12196
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>