cxl/test: Add missing vmalloc.h for tools/testing/cxl/test/mem.c

tools/testing/cxl/test/mem.c uses vmalloc() and vfree() but does not
include linux/vmalloc.h. Kernel v6.10 made changes that causes the
currently included headers not depend on vmalloc.h and therefore
mem.c can no longer compile. Add linux/vmalloc.h to fix compile
issue.

  CC [M]  tools/testing/cxl/test/mem.o
tools/testing/cxl/test/mem.c: In function ‘label_area_release’:
tools/testing/cxl/test/mem.c:1428:9: error: implicit declaration of function ‘vfree’; did you mean ‘kvfree’? [-Werror=implicit-function-declaration]
 1428 |         vfree(lsa);
      |         ^~~~~
      |         kvfree
tools/testing/cxl/test/mem.c: In function ‘cxl_mock_mem_probe’:
tools/testing/cxl/test/mem.c:1466:22: error: implicit declaration of function ‘vmalloc’; did you mean ‘kmalloc’? [-Werror=implicit-function-declaration]
 1466 |         mdata->lsa = vmalloc(LSA_SIZE);
      |                      ^~~~~~~
      |                      kmalloc

Fixes: 7d3eb23c

 ("tools/testing/cxl: Introduce a mock memory device + driver")
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://lore.kernel.org/r/20240528225551.1025977-1-dave.jiang@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>