🩹 Use #pragma once in pins files

Marlin/src/pins/mega/pins_PICA.h

@@ -19,6 +19,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 /**
  * Arduino Mega with PICA pin assignments

Marlin/src/pins/mega/pins_PICAOLD.h

@@ -19,6 +19,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  *
  */
+#pragma once
 
 #define HEATER_0_PIN                          9   // E0
 #define HEATER_1_PIN                         10   // E1