Ensure to set _GNU_SOURCE for POSIX compliance with signals

The systemd bus headers aren't setting a POSIX compliance level so we
define the catch-all to ensure the build doesn't error out, as seen with
issue #3. This is due to the reliance of `siginfo_t`, which requires a
minimum POSIX level of `199309L` when using glibc.

Signed-off-by: Ikey Doherty <ikey@solus-project.com>

lib/client_impl.c

@@ -28,6 +28,9 @@ ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGE.
 
  */
+
+#define _GNU_SOURCE
+
 #include <dlfcn.h>
 #include <errno.h>
 #include <stdio.h>