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)
 }
 

wgengine/userspace.go

@@ -23,6 +23,7 @@ import (
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/drive"
 	"tailscale.com/envknob"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/health"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
@@ -1652,6 +1653,9 @@ var (
 )
 
 func (e *userspaceEngine) InstallCaptureHook(cb packet.CaptureCallback) {
+	if !buildfeatures.HasCapture {
+		return
+	}
 	e.tundev.InstallCaptureHook(cb)
 	e.magicConn.InstallCaptureHook(cb)
 }

wgengine/watchdog.go

@@ -15,6 +15,7 @@ import (
 	"time"
 
 	"tailscale.com/envknob"
+	"tailscale.com/feature/buildfeatures"
 	"tailscale.com/ipn/ipnstate"
 	"tailscale.com/net/dns"
 	"tailscale.com/net/packet"
@@ -163,6 +164,9 @@ func (e *watchdogEngine) Done() <-chan struct{} {
 }
 
 func (e *watchdogEngine) InstallCaptureHook(cb packet.CaptureCallback) {
+	if !buildfeatures.HasCapture {
+		return
+	}
 	e.wrap.InstallCaptureHook(cb)
 }