Fix compilation on newer Linux systems

Defining _GNU_SOURCE will set the appropriate POSIX levels required to
use the `_MAX`, `kill`, `realpath`, etc, set of POSIX family functions
and defines.

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

daemon/gamemode.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 "gamemode.h"
 #include "governors.h"
 #include "logging.h"