Fix: field-path.hpp: add missing `shared-obj.hpp` include

Fix: field-path.hpp: add missing `shared-obj.hpp` include

gcc 9.3 emits the following warning when including the `field-path.hpp`
file by itself:
  ../../../../src/cpp-common/bt2/field-path.hpp:180:5: error: return type ‘using Shared = class bt2::internal::SharedObj<bt2::ConstFieldPath, const bt_field_path, bt2::internal::FieldPathRefFuncs>’ {aka ‘class bt2::internal::SharedObj<bt2::ConstFieldPath, const bt_field_path, bt2::internal::FieldPathRefFuncs>’} is incomplete
    180 |     {
        |     ^
  ../../../../src/cpp-common/bt2/field-path.hpp: In member function ‘void bt2::ConstFieldPath::shared() const’:
  ../../../../src/cpp-common/bt2/field-path.hpp:181:29: error: too many initializers for ‘bt2::ConstFieldPath::Shared’ {aka ‘bt2::internal::SharedObj<bt2::ConstFieldPath, const bt_field_path, bt2::internal::FieldPathRefFuncs>’}
    181 |         return Shared {*this};
        |                             ^

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: If18d48f16eb6ecb5b14e12a9c8314392c3f3698e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/7604
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10786
CI-Build: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>