kbd: Suppress keys without mappings

Don't warn if a key without a mapping is pressed - it's known that
some keys aren't mapped to keycodes.  Suppress these keys instead of
sending 0x0000 to the keyboard buffer - as 0x0000 can confuse some
programs.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/kbd.c b/src/kbd.c
index cc41025..203402a 100644
--- a/src/kbd.c
+++ b/src/kbd.c
@@ -553,9 +553,8 @@
             else
                 keycode = (keycode & 0xff00) | 0xe0;
         }
-        if (!keycode)
-            dprintf(1, "KBD: keycode is zero?\n");
-        enqueue_key(keycode);
+        if (keycode)
+            enqueue_key(keycode);
         break;
     }
     flags2 &= ~KF2_LAST_E0;