tools/testing/nvdimm: Make symbol '__nfit_test_ioremap' static

tools/testing/nvdimm: Make symbol '__nfit_test_ioremap' static

The sparse tool complains as follows:

tools/testing/nvdimm/test/iomap.c:65:14: warning:
 symbol '__nfit_test_ioremap' was not declared. Should it be static?

This symbol is not used outside of iomap.c, so this
commit marks it static.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zou Wei <zou_wei@huawei.com>
Link: https://lore.kernel.org/r/1618904867-25275-1-git-send-email-zou_wei@huawei.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>