more IWYU fixes

I'm starting to wonder if IWYU is worth the effort. Nonetheless, this
makes it lint clean on macOS and reduces the number of warnings on
FreeBSD and Linux.

src/builtin_commandline.cpp

@@ -3,6 +3,7 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <stddef.h>
 #include <stdlib.h>
 #include <wchar.h>