scripts/kallsyms: Enable error messages while hush up unnecessary warnings

As no error was handled, we wouldn't be able to know when an error does
occur. The fix preserves error messages while it doesn't let unnecessary
compiling warnings show up.

Signed-off-by: Jean Sacren <sakiwit@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>