[S390] Export uaccess as non-gpl symbol.

Commit c1821c2e

 introduced the
uaccess structure that is used to select the correct set of user
copy functions for the different execution modes (standard vs.
noexec vs. z9 optimized user copy). The uaccess symbol is exported
with EXPORT_SYMBOL_GPL. This breaks all non-gpl modules that use
user copy. To make them work again change the export to
EXPORT_SYMBOL.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>