fixdep: do not parse *.rlib, *.rmeta, *.so

fixdep is designed only for parsing text files. read_file() appends
a terminating null byte ('\0') and parse_config_file() calls strstr()
to search for CONFIG options.

rustc outputs *.rlib, *.rmeta, *.so to dep-info. fixdep needs them in
the dependency, but there is no point in parsing such binary files.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Tested-by: Miguel Ojeda <ojeda@kernel.org>