Fix compile error in bootloader_message_twrp on 6.0 tree

Change-Id: Ifcfff60fab1cd73b524cd521a7094c344f8e9e1d
diff --git a/bootloader_message_twrp/bootloader_message.cpp b/bootloader_message_twrp/bootloader_message.cpp
index 55d3a5d..6719739 100644
--- a/bootloader_message_twrp/bootloader_message.cpp
+++ b/bootloader_message_twrp/bootloader_message.cpp
@@ -341,7 +341,7 @@
                 (*argv)[*argc] = strdup(arg);
             }
             printf("Got arguments from boot message\n");
-        } else if (boot.recovery[0] != 0 && boot.recovery[0] != 255) {
+        } else if (boot.recovery[0] != 0 && boot.recovery[0] != (char)255) {
             printf("Bad boot message\n\"%.20s\"\n", boot.recovery);
         }
     }