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/firstboot/firstboot.c

@@ -29,7 +29,6 @@
 #include "hostname-util.h"
 #include "image-policy.h"
 #include "kbd-util.h"
-#include "label.h"
 #include "label-util.h"
 #include "libcrypt-util.h"
 #include "locale-setup.h"