selftests: kallsyms: add MODULE_DESCRIPTION

The newly added test script creates modules that are lacking
a description line in order to build cleanly:

WARNING: modpost: missing MODULE_DESCRIPTION() in lib/tests/module/test_kallsyms_a.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/tests/module/test_kallsyms_b.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/tests/module/test_kallsyms_c.o
WARNING: modpost: missing MODULE_DESCRIPTION() in lib/tests/module/test_kallsyms_d.o

Fixes: 84b4a51f ("selftests: add new kallsyms selftests")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>