brlock: fix build with KVM_BRLOCK_DEBUG

Build breaks when using KVM_BRLOCK_DEBUG because the header was seamingly
conceived to be included in a single .c file...

Fix this by moving the definition of the read/write lock into the kvm
struct.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>