Fix wrong header include on interactive gatttool

This wrong include is triggering compiler warning: implicit declaration
of function 'strdup'.
diff --git a/attrib/interactive.c b/attrib/interactive.c
index 83a0778..a98fe79 100644
--- a/attrib/interactive.c
+++ b/attrib/interactive.c
@@ -20,7 +20,7 @@
  *  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
  *
  */
-#include <strings.h>
+#include <string.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <glib.h>