refactor: Add clang-tidy checks and simplify codebase

include/x11.h

@@ -1,4 +1,5 @@
-#pragma once
+#ifndef X11_H
+#define X11_H
 
 #include <xcb/xcb.h>
 
@@ -8,3 +9,5 @@ x11_connection* x11_connection_open(void);
 void x11_connection_close(x11_connection* const connection);
 int x11_set_root_name(x11_connection* const connection,
                       const char* const name);
+
+#endif  // X11_H