Add include guards to some headers

Marlin/src/HAL/HAL_ESP32/FlushableHardwareSerial.h

@@ -19,6 +19,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 #ifdef ARDUINO_ARCH_ESP32
 

Marlin/src/HAL/HAL_ESP32/SanityCheck.h

@@ -19,6 +19,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 #if ENABLED(EMERGENCY_PARSER)
   #error "EMERGENCY_PARSER is not yet implemented for ESP32. Disable EMERGENCY_PARSER to continue."

Marlin/src/HAL/HAL_ESP32/servotimers.h

@@ -19,3 +19,4 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
  */
+#pragma once