patch 8.2.3250: MS-Windows: cannot build with libsodium

Problem:    MS-Windows: cannot build with libsodium.
Solution:   Change FEAT_SODIUM into HAVE_SODIUM. (Christian Brabandt,
            closes #8668, closes #8663)
diff --git a/src/Make_mvc.mak b/src/Make_mvc.mak
index 8c8e71a..18b5a88 100644
--- a/src/Make_mvc.mak
+++ b/src/Make_mvc.mak
@@ -395,7 +395,7 @@
 
 !if "$(SODIUM)" != "no"
 SOD_INC		= /I "$(SODIUM)\include"
-SOD_DEFS	= -DFEAT_SODIUM
+SOD_DEFS	= -DHAVE_SODIUM
 SOD_LIB		= $(SOD_LIB)\libsodium.lib
 !endif