linux/export.h: rename 'sec' argument to 'license'

Now, EXPORT_SYMBOL() is populated in two stages. In the first stage,
all of EXPORT_SYMBOL/EXPORT_SYMBOL_GPL go into the same section,
'.export_symbol'.

'sec' does not make sense any more. Rename it to 'license'.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>