build: add missing includes to header files

This change adds necessary `#include` directives that were previously
omitted in some header files. Without these includes, compilation
could fail or produce undefined behavior when the headers are used
in isolation (i.e., before their dependencies are included elsewhere).

Ensures better modularity and reliability of header usage.

src/ui/titlebar.h

@@ -10,6 +10,8 @@
 #ifndef UI_TITLEBAR_H
 #define UI_TITLEBAR_H
 
+#include "glib.h"
+
 void create_title_bar(void);
 void free_title_bar(void);
 void title_bar_update_virtual(void);