tmpfile-util: Drop libgen.h

tmpfile-util: Drop libgen.h

We don't want the version of basename() that may leak memory - we want
the sane one. I remembered to add the missing.h while editing commit
aae48bc9f73a ("depmod: add tmpfile-util to generate temporary file") to
merge it, but completely forgot to remove the libgen.h. Fix it now.

Fixes: aae48bc9f73a ("depmod: add tmpfile-util to generate temporary file")
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/335