libc: Update includes

Reflect the changes in the structure of libc.

NB: the include of stdarg.h in nvm_results_helpers.c is not in alphabetical
order because it needs to be included before stdio.h. Fixing this would require
further changes.

Change-Id: I07f62a3450802833408ff3e1f950fd3b643e5e33
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>