modpost: do not warn about missing MODULE_DESCRIPTION() for vmlinux.o

Building with W=1 incorrectly emits the following warning:

  WARNING: modpost: missing MODULE_DESCRIPTION() in vmlinux.o

This check should apply only to modules.

Fixes: 1fffe7a3 ("script: modpost: emit a warning when the description is missing")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

scripts/mod/modpost.c

@@ -1647,10 +1647,11 @@ static void read_symbols(const char *modname)
 			namespace = get_next_modinfo(&info, "import_ns",
 						     namespace);
 		}
-	}
 
 		if (extra_warn && !get_modinfo(&info, "description"))
 			warn("missing MODULE_DESCRIPTION() in %s\n", modname);
+	}
+
 	for (sym = info.symtab_start; sym < info.symtab_stop; sym++) {
 		symname = remove_dot(info.strtab + sym->st_name);