dpif-netdev-perf: Fix linker unresolved symbols on Windows

dpif-netdev-perf: Fix linker unresolved symbols on Windows

MSVC complains:
"libopenvswitch.lib(dpif-netdev.obj) : error LNK2019: unresolved external
symbol pmd_perf_start_iteration referenced in function pmd_thread_main
libopenvswitch.lib(dpif-netdev.obj) : error LNK2019: unresolved external
symbol pmd_perf_end_iteration referenced in function pmd_thread_main"

Remove inline keyword from the declaration of:
`pmd_perf_start_iteration` and `pmd_perf_end_iteration`

More on the subject:
https://docs.microsoft.com/en-us/cpp/error-messages/tool-errors/function-inlining-problems

Fixes: broken build on Windows
Signed-off-by: Alin Gabriel Serdean <aserdean@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>