lib/nlattr.c: move EXPORT_SYMBOL after functions

Fix some checkpatch warnings:
WARNING: EXPORT_SYMBOL(foo); should immediately follow its function/variable

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Pablo Neira <pablo@netfilter.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>