fix: compilation error with gcc 11

../src/modules/network.cpp:22:6: error: ‘optional’ in namespace ‘std’ does not name a template type
   22 | std::optional<unsigned long long> read_netstat(std::string_view category, std::string_view key) {
      |      ^~~~~~~~
../src/modules/network.cpp:7:1: note: ‘std::optional’ is defined in header ‘<optional>’; did you forget to ‘#include <optional>’?
    6 | #include "util/format.hpp"
  +++ |+#include <optional>
    7 | #ifdef WANT_RFKILL

src/modules/network.cpp

@@ -3,6 +3,7 @@
 #include <sys/eventfd.h>
 #include <fstream>
 #include <cassert>
+#include <optional>
 #include "util/format.hpp"
 #ifdef WANT_RFKILL
 #include "util/rfkill.hpp"