ofproto-dpif: Fix incorrect printf format specifiers.

ofproto-dpif: Fix incorrect printf format specifiers.

    cc1: warnings being treated as errors
    ../ofproto/ofproto-dpif.c: In function ‘dpif_show_backer’:
    ../ofproto/ofproto-dpif.c:8305: error: format ‘%llu’ expects type ‘long
    long unsigned int’, but argument 4 has type ‘size_t’
    ../ofproto/ofproto-dpif.c:8305: error: format ‘%llu’ expects type ‘long
    long unsigned int’, but argument 5 has type ‘size_t’

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>