all: use buildfeatures.HasCapture const in a handful of places

Help out the linker's dead code elimination.

Updates #12614

Change-Id: I6c13cb44d3250bf1e3a01ad393c637da4613affb
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock.go

@@ -890,6 +890,9 @@ func deregisterMetrics(m *metrics) {
 // can be called with a nil argument to uninstall the capture
 // hook.
 func (c *Conn) InstallCaptureHook(cb packet.CaptureCallback) {
+	if !buildfeatures.HasCapture {
+		return
+	}
 	c.captureHook.Store(cb)
 }