mm: Fix a build breakage in memcontrol-v1.c

While adding a deprecation message, fd4fd0a8 ("mm: Add transformation
message for per-memcg swappiness") missed the semicolon after the new
pr_info_once() statement causing build breakage when CONFIG_MEMCG_V1 is
enabled. Fix it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Fixes: fd4fd0a8 ("mm: Add transformation message for per-memcg swappiness")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202503120710.guZkJx0h-lkp@intel.com/