Improve the include situation

Slightly clean up header inclusion with the help of clang's module system.
It's better for files to include the things you need directly.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/win.c

@@ -10,6 +10,7 @@
 
 #include "compiler.h"
 #include "common.h"
+#include "config.h"
 #include "compton.h"
 #include "c2.h"
 #include "x.h"