Revert "remove import that causes issues"

This reverts commit 3d9d672.

Author: withinboredom

internal/extgen/templates/extension.go.tpl

@@ -5,6 +5,7 @@ package {{.PackageName}}
 #include "{{.BaseName}}.h"
 */
 import "C"
+import "runtime/cgo"
 {{- range .Imports}}
 import {{.}}
 {{- end}}