baum: Fix build with debugging enabled

cur and buf are pointers, so the difference is a ptrdiff_t

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

backends/baum.c

@@ -303,7 +303,7 @@ static int baum_eat_packet(BaumDriverState *baum, const uint8_t *buf, int len)
                 return 0;
             cur++;
         }
-        DPRINTF("Dropped %d bytes!\n", cur - buf);
+        DPRINTF("Dropped %td bytes!\n", cur - buf);
     }
 
 #define EAT(c) do {\