examples/ntb: fix build

examples/ntb: fix build

lcore indexes can't be represented with uint8_t.
Seen on armv8 target:

examples/ntb/ntb_fwd.c: In function ‘cmd_quit_parsed’:
arm64-armv8a-linux-gcc/include/rte_lcore.h:296:8:
error: comparison is always true due to limited range of data type
[-Werror=type-limits]
       i<RTE_MAX_LCORE;      \
        ^
examples/ntb/ntb_fwd.c:164:2: note: in expansion of
macro ‘RTE_LCORE_FOREACH_SLAVE’
  RTE_LCORE_FOREACH_SLAVE(lcore_id) {
  ^~~~~~~~~~~~~~~~~~~~~~~

Fixes: 5194299d6ef5 ("examples/ntb: support more functions")

Signed-off-by: David Marchand <david.marchand@redhat.com>