ltmain.in: add parentheses to support values with spaces

The variable may contain a value with spaces that cause errors due
to the standard splitting of command line arguments by linux shells.
See https://github.com/autotools-mirror/libtool/pull/4/files.

Reported: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=79826

* build-aux/ltmain.in: Add missing quotes around darwin_orig_dir.
diff --git a/build-aux/ltmain.in b/build-aux/ltmain.in
index 0f2c81f..bb72916 100644
--- a/build-aux/ltmain.in
+++ b/build-aux/ltmain.in
@@ -3505,7 +3505,7 @@
 	    $RM -rf unfat-$$
 	    cd "$darwin_orig_dir"
 	  else
-	    cd $darwin_orig_dir
+	    cd "$darwin_orig_dir"
 	    func_extract_an_archive "$my_xdir" "$my_xabs"
 	  fi # $darwin_arches
 	} # !$opt_dry_run