refactor: fix IWYU mapping file and use IWYU (#21802)

Also add the EXITFREE definition to main_lib rather than the nvim target, as the header generation needs the EXITFREE flag to work properly.

src/nvim/runtime.c

@@ -9,7 +9,6 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <inttypes.h>
-#include <stddef.h>
 #include <stdio.h>
 #include <string.h>
 #include <uv.h>