🐛 Fix M876 without emergency parser

Fix regression from 1fb2fffdbf

Marlin/src/gcode/gcode.cpp

         case 108: M108(); break;                                  // M108: Cancel Waiting
         case 112: M112(); break;                                  // M112: Full Shutdown
         case 410: M410(); break;                                  // M410: Quickstop - Abort all the planned moves.
-        TERN_(HOST_PROMPT_SUPPORT, case 876:)                     // M876: Handle Host prompt responses
+        #if ENABLED(HOST_PROMPT_SUPPORT)
+          case 876: M876(); break;                                // M876: Handle Host prompt responses
+        #endif
       #else
         case 108: case 112: case 410:
         TERN_(HOST_PROMPT_SUPPORT, case 876:)