libsemanage: make lang_ext parameter const in semanage_direct_write_langext()

When building with "clang -Wwrite-strings", the compiler reports the
following warnings:

    direct_api.c:1030:46: error: passing 'const char [4]' to parameter
    of type 'char *' discards qualifiers
    [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
                    status = semanage_direct_write_langext(sh, "cil", modinfo);
                                                               ^~~~~
    direct_api.c:898:11: note: passing argument to parameter 'lang_ext'
    here
                                    char *lang_ext,
                                          ^
    direct_api.c:1030:46: error: passing 'const char [4]' to parameter
    of type 'char *' discards qualifiers
    [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
                    status = semanage_direct_write_langext(sh, "cil", modinfo);
                                                               ^~~~~
    direct_api.c:898:11: note: passing argument to parameter 'lang_ext'
    here
                                    char *lang_ext,
                                          ^

Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>
1 file changed