Include fd_stream in command.hpp (#2533)

On older gcc versions, the incomplete type can lead to a compilation
error because unique_ptr requires the complete type for its destructor.

include/utils/command.hpp

@@ -4,13 +4,11 @@
 #include "components/logger.hpp"
 #include "components/types.hpp"
 #include "errors.hpp"
+#include "utils/file.hpp"
 #include "utils/functional.hpp"
 
 POLYBAR_NS
 
-template <typename T>
-class fd_stream;
-
 DEFINE_ERROR(command_error);
 
 /**