Fixed some build warnings

[zjeffer]

While compiling Waybar, I had these warnings:

In file included from ../src/modules/network.cpp:1:
../include/modules/network.hpp: In constructor ‘waybar::modules::Network::Network(const std::string&, const Json::Value&)’:
../include/modules/network.hpp:87:16: warning: ‘waybar::modules::Network::rfkill_’ will be initialized after [-Wreorder]
   87 |   util::Rfkill rfkill_;
      |                ^~~~~~~
../include/modules/network.hpp:81:9: warning:   ‘float waybar::modules::Network::frequency_’ [-Wreorder]
   81 |   float frequency_;
      |         ^~~~~~~~~~
../src/modules/network.cpp:80:1: warning:   when initialized here [-Wreorder]
   80 | waybar::modules::Network::Network(const std::string &id, const Json::Value &config)
      | ^~~~~~
[141/150] Compiling C++ object waybar.p/src_util_rfkill.cpp.o
../src/util/rfkill.cpp: In member function ‘bool waybar::util::Rfkill::on_event(Glib::IOCondition)’:
../src/util/rfkill.cpp:66:13: warning: comparison of integer expressions of different signedness: ‘ssize_t’ {aka ‘long int’} and ‘long unsigned int’ [-Wsign-compare]
   66 |     if (len < RFKILL_EVENT_SIZE_V1) {
      |             ^

This PR fixes these warnings.

[Alexays]

Thanks!