um: Remove straying parenthesis

Commit e3a33af8 ("um: fix and optimize xor select template for CONFIG64 and timetravel mode")
caused a build regression when CONFIG_XOR_BLOCKS and CONFIG_UML_TIME_TRAVEL_SUPPORT
are selected.
Fix it by removing the straying parenthesis.

Cc: stable@vger.kernel.org
Fixes: e3a33af8 ("um: fix and optimize xor select template for CONFIG64 and timetravel mode")
Signed-off-by: Benjamin Beichler <benjamin.beichler@uni-rostock.de>
[rw: Added commit message]
Signed-off-by: Richard Weinberger <richard@nod.at>