Add missing import

GoCompile: missing strict dependencies:
	/tmpfs/tmp/bazel/sandbox/linux-sandbox/1744/execroot/__main__/runsc/main.go:
	import of "gvisor.googlesource.com/gvisor/runsc/specutils"

This was broken in 210995199.

PiperOrigin-RevId: 211086595
Change-Id: I166b9a2ed8e4d6e624def944b720190940d7537c

runsc/BUILD

@@ -45,6 +45,7 @@ go_binary(
         "//pkg/log",
         "//runsc/boot",
         "//runsc/cmd",
+        "//runsc/specutils",
         "@com_github_google_subcommands//:go_default_library",
     ],
 )