patch 8.0.1252: incomplete translations makefile for MinGW/Cygwin

Problem:    Incomplete translations makefile for MinGW/Cygwin.
Solution:   Add missing source files.  Make it work with msys2's bash. (Ken
            Takata)
diff --git a/src/po/Make_mvc.mak b/src/po/Make_mvc.mak
index 9deba79..85013f4 100644
--- a/src/po/Make_mvc.mak
+++ b/src/po/Make_mvc.mak
@@ -117,7 +117,7 @@
 all: $(MOFILES)
 
 files:
-	$(LS) $(LSFLAGS) ..\*.c ..\if_perl.xs ..\globals.h > .\files
+	$(LS) $(LSFLAGS) ..\*.c ..\if_perl.xs ..\GvimExt\gvimext.cpp ..\globals.h ..\if_py_both.h > .\files
 
 first_time: files
 	set OLD_PO_FILE_INPUT=yes