modpost: do not call get_modinfo() for vmlinux(.o)

The three calls of get_modinfo() ("license", "import_ns", "version")
always return NULL for vmlinux(.o) because the built-in module info is
prefixed with __MODULE_INFO_PREFIX.

It is harmless to call get_modinfo(), but there is no point to search
for what apparently does not exist.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>