tools/testing/cxl: Use DEFINE_STATIC_SRCU()

Starting with commit:

95433f72

 ("srcu: Begin offloading srcu_struct fields to srcu_update")

...it is no longer possible to do:

static DEFINE_SRCU(x)

Switch to DEFINE_STATIC_SRCU(x) to fix:

tools/testing/cxl/test/mock.c:22:1: error: duplicate ‘static’
   22 | static DEFINE_SRCU(cxl_mock_srcu);
      | ^~~~~~

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/168392709546.1135523.10424917245934547117.stgit@dwillia2-xfh.jf.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>