tools/nolibc: Fix build of stdio.h due to header ordering

When we added fd based file streams we created references to STx_FILENO in
stdio.h but these constants are declared in unistd.h which is the last file
included by the top level nolibc.h meaning those constants are not defined
when we try to build stdio.h. This causes programs using nolibc.h to fail
to build.

Reorder the headers to avoid this issue.

Fixes: d449546c957f ("tools/nolibc: implement fd-based FILE streams")
Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>