patch 8.1.2082: some files have a weird name to fit in 8.3 characters

Problem:    Some files have a weird name to fit in 8.3 characters.
Solution:   Use a nicer names.
diff --git a/src/proto.h b/src/proto.h
index 6d3b83d..4c419c6 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -182,7 +182,7 @@
 # include "ops.pro"
 # include "option.pro"
 # include "optionstr.pro"
-# include "popupmnu.pro"
+# include "popupmenu.pro"
 # if defined(FEAT_PROFILE) || defined(FEAT_RELTIME)
 # include "profiler.pro"
 # endif