fix(linux_introspection): declare pkg-config as buildtool_depend

CMakeLists.txt uses find_package(PkgConfig REQUIRED) to locate
libsystemd via pkg_check_modules, but package.xml did not declare
pkg-config as a buildtool dependency. On clean buildfarm agents
rosdep only installs declared dependencies, so the pkg-config
binary was missing and CMake configure failed. Local dev
environments typically have pkg-config installed system-wide,
which masked the issue.

Closes #352

Author: bburda

src/ros2_medkit_discovery_plugins/ros2_medkit_linux_introspection/package.xml

@@ -9,6 +9,7 @@
 
   <buildtool_depend>ament_cmake</buildtool_depend>
   <buildtool_depend>ros2_medkit_cmake</buildtool_depend>
+  <buildtool_depend>pkg-config</buildtool_depend>
 
   <depend>ros2_medkit_gateway</depend>
   <depend>nlohmann-json-dev</depend>