selftests: ublk: kublk: build with -Werror iff WERROR!=0

selftests: ublk: kublk: build with -Werror iff WERROR!=0

Compiler warnings can catch bugs at compile time; thus, heeding them is
usually a good idea. Turn warnings into errors by default for the kublk
build so that anyone making changes is forced to heed them. Compiler
warnings can also sometimes produce annoying false positives, so provide
a flag WERROR that the developer can use as follows to have the build
and selftests run go through even if there are warnings:

make WERROR=0 TARGETS=ublk kselftest

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250429-ublk_selftests-v2-1-e970b6d9e4f4@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>