Use PGM_P for PSTR pointers (#11977)

Marlin/src/Marlin.cpp

@@ -599,7 +599,7 @@ void idle(
  * Kill all activity and lock the machine.
  * After this the machine will need to be reset.
  */
-void kill(const char* lcd_msg) {
+void kill(PGM_P lcd_msg) {
   SERIAL_ERROR_START();
   SERIAL_ERRORLNPGM(MSG_ERR_KILLED);