🩹 Fix ExtUI build with Host Keepalive disabled (#23898)

Marlin/src/lcd/extui/ui_api.cpp

     return !thermalManager.tooColdToExtrude(extruder - E0);
   }
 
-  GcodeSuite::MarlinBusyState getHostKeepaliveState() { return TERN0(HOST_KEEPALIVE_FEATURE, gcode.busy_state); }
-  bool getHostKeepaliveIsPaused() { return TERN0(HOST_KEEPALIVE_FEATURE, gcode.host_keepalive_is_paused()); }
+  #if ENABLED(HOST_KEEPALIVE_FEATURE)
+    GcodeSuite::MarlinBusyState getHostKeepaliveState() { return gcode.busy_state; }
+    bool getHostKeepaliveIsPaused() { return gcode.host_keepalive_is_paused(); }
+  #endif
 
   #if HAS_SOFTWARE_ENDSTOPS
     bool getSoftEndstopState() { return soft_endstop._enabled; }

Marlin/src/lcd/extui/ui_api.h

   void injectCommands(char * const);
   bool commandsInQueue();
 
-  GcodeSuite::MarlinBusyState getHostKeepaliveState();
-  bool getHostKeepaliveIsPaused();
+  #if ENABLED(HOST_KEEPALIVE_FEATURE)
+    GcodeSuite::MarlinBusyState getHostKeepaliveState();
+    bool getHostKeepaliveIsPaused();
+  #endif
 
   bool isHeaterIdle(const heater_t);
   bool isHeaterIdle(const extruder_t);