add two missing includes

lib/formats/json_reserve.c

@@ -20,6 +20,8 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *********************************************************************************/
 
+#include <string.h>
+
 #include <villas/plugin.h>
 #include <villas/sample.h>
 #include <villas/node.h>

lib/hooks/print.c

@@ -24,6 +24,8 @@
  * @{
  */
 
+#include <string.h>
+
 #include <villas/hook.h>
 #include <villas/plugin.h>
 #include <villas/path.h>