sdcard.cpp: added compiler directive

src/sdcard.cpp

@@ -1,5 +1,7 @@
 // routines for writing data to an SD-card, if present
 
+#if HAS_SDCARD)
+
 // Local logging tag
 static const char TAG[] = __FILE__;
 
@@ -64,4 +66,6 @@ void createFile(void) {
     }
   }
   return;
-}
+}
+
+#endif // (HAS_SDCARD)