Add include guards to some headers

Marlin/src/HAL/HAL_DUE/G2_PWM.h

@@ -19,6 +19,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 /**
  * This module is stripped down version of the LPC1768_PWM.h file from

Marlin/src/HAL/HAL_DUE/SanityCheck.h

@@ -19,6 +19,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 /**
  * Test Arduino Due specific configuration values for errors at compile-time.