liblockdep: Fix 'set but not used' warnings

liblockdep: Fix 'set but not used' warnings

liblockdep defines trivial macros for working with interrupt flags, as
interrupts are never disabled in userland.  This results in warnings
from gcc when -Wunused-but-set-variable is enabled, and it is enabled
by -Wall.  Fix this by evaluating the flags parameter and casting it to
void.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name liblockdep-fix-set-but-not-used-warnings.patch