mm: page_alloc: set sysctl_lowmem_reserve_ratio storage-class-specifier to static

smatch reports
mm/page_alloc.c:247:5: warning: symbol
  'sysctl_lowmem_reserve_ratio' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static

Link: https://lkml.kernel.org/r/20230518141119.927074-1-trix@redhat.com
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>