bpcmitm_main: Add missing header guard

Prevents potential inclusion issues.

stratosphere/ams_mitm/source/bpc_mitm/bpcmitm_main.hpp

@@ -14,6 +14,8 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#pragma once
+
 #include <cstdlib>
 #include <cstdint>
 #include <cstring>