devtools: ignore Linux-style fallthrough warning

devtools: ignore Linux-style fallthrough warning

The PREFER_FALLTHROUGH check warns if a passthrough comment is found
because, in the kernel, the special macro "fallthrough" is preferred.

Since that keyword is not defined in DPDK, ignore the warning.

Ignoring this check does not affect the MISSING_BREAK check that will
warn if a switch case/default is not preceded by break or a fallthrough
comment.

Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Chenbo Xia <chenbo.xia@intel.com>