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>

lib/dpif-netdev-perf.c

@@ -440,7 +440,7 @@ pmd_perf_stats_clear(struct pmd_perf_stats *s)
 
 /* Functions recording PMD metrics per iteration. */
 
-inline void
+void
 pmd_perf_start_iteration(struct pmd_perf_stats *s)
 OVS_REQUIRES(s->stats_mutex)
 {
@@ -462,7 +462,7 @@ OVS_REQUIRES(s->stats_mutex)
     }
 }
 
-inline void
+void
 pmd_perf_end_iteration(struct pmd_perf_stats *s, int rx_packets,
                        int tx_packets, bool full_metrics)
 {