clang-tidy: Fix all remaining misc-include-cleaner violations

- Remove unused includes
- Add common false positive headers to misc-include-cleaner.IgnoreHeaders
- Add IWYU pragma keep for uncommon false positive headers

src/sysext/sysext.c

@@ -32,7 +32,7 @@
 #include "hashmap.h"
 #include "image-policy.h"
 #include "initrd-util.h"
-#include "label-util.h"
+#include "label-util.h"                 /* IWYU pragma: keep */
 #include "log.h"
 #include "loop-util.h"
 #include "main-func.h"