AP_Filesystem: add HAL_UART_STATS_ENABLED to disable stats gathering

libraries/AP_Filesystem/AP_Filesystem_Sys.cpp

@@ -105,9 +105,11 @@ int AP_Filesystem_Sys::open(const char *fname, int flags)
     if (strcmp(fname, "memory.txt") == 0) {
         hal.util->mem_info(*r.str);
     }
+#if HAL_UART_STATS_ENABLED
     if (strcmp(fname, "uarts.txt") == 0) {
         hal.util->uart_info(*r.str);
     }
+#endif
 #if HAL_CANMANAGER_ENABLED
     if (strcmp(fname, "can_log.txt") == 0) {
         AP::can().log_retrieve(*r.str);