Fix compile error with watch_bed_next_ms

Marlin/temperature.cpp

   millis_t Temperature::watch_heater_next_ms[HOTENDS] = { 0 };
 #endif
 
-#if ENABLED(THERMAL_PROTECTION_HOTENDS) && WATCH_BED_TEMP_PERIOD > 0
+#if ENABLED(THERMAL_PROTECTION_BED) && WATCH_BED_TEMP_PERIOD > 0
   int Temperature::watch_target_bed_temp = 0;
   millis_t Temperature::watch_bed_next_ms = 0;
 #endif

Marlin/temperature.h

       static millis_t watch_heater_next_ms[HOTENDS];
     #endif
 
-    #if ENABLED(THERMAL_PROTECTION_HOTENDS) && WATCH_BED_TEMP_PERIOD > 0
+    #if ENABLED(THERMAL_PROTECTION_BED) && WATCH_BED_TEMP_PERIOD > 0
       static int watch_target_bed_temp;
       static millis_t watch_bed_next_ms;
     #endif