relational: Fix signbit

The vector components were mistakenly using () instead of {}, which caused
all but the last vector component to be dropped on the floor.

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Jeroen Ketema <j.ketema@imperial.ac.uk>

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@211733 91177308-0d34-0410-b5e6-96231b3b80d8