Add bindings for ligature oe to iso-transl-char-map

* lisp/international/iso-transl.el (iso-transl-char-map): Add bindings
for small and capital ligature oe.  (Bug#23420)

lisp/international/iso-transl.el

@@ -177,6 +177,8 @@
     ("c"    . [?¢])
     ("*o"   . [?°])
     ("o"    . [?°])
+    ("Oe"   . [?œ])
+    ("OE"   . [?Œ])
     ("*u"   . [?µ])
     ("u"    . [?µ])
     ("*m"   . [?µ])