kbuild: in the section mismatch check try harder to find symbols

kbuild: in the section mismatch check try harder to find symbols

When searching for symbols the only check performed was if
offset equals st_value. Adding an additional check to see if st_name
points t a valid name made us sort out a few more false positives and
let us report more correct names in warnings.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>