lib: test_hmm: add missing MODULE_DESCRIPTION() macro

make allmodconfig && make W=1 C=1 reports:
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/test_hmm.o

Add the missing invocation of the MODULE_DESCRIPTION() macro.

Link: https://lkml.kernel.org/r/20240531-lib-md-test_hmm-v1-1-e4aa17daa57b@quicinc.com


Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>