Perfstats are not supported on Windows

cmd/syncthing/main.go

@@ -127,7 +127,8 @@ The following enviroment variables are interpreted by syncthing:
  STHEAPPROFILE Write heap profiles to heap-$pid-$timestamp.pprof each time
                heap usage increases.
 
- STPERFSTATS   Write running performance statistics to perf-$pid.csv.
+ STPERFSTATS   Write running performance statistics to perf-$pid.csv. Not
+               supported on Windows.
 
  STDEADLOCKTIMEOUT  Alter deadlock detection timeout (seconds; default 1200).`
 )

cmd/syncthing/perfstats_unix.go

@@ -2,6 +2,8 @@
 // All rights reserved. Use of this source code is governed by an MIT-style
 // license that can be found in the LICENSE file.
 
+// +build !windows
+
 package main
 
 import (