Only check header for non-generated files

For generated files, it is enough to check the headers of the
files they are generated from. Moreover, the generated files
have the shebang-line of the target system, which might be
different from the generic one checked for.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Niklas Hambuechen <niklash@google.com>