patch 8.0.1157: compiler warning on MS-Windows

Problem:    Compiler warning on MS-Windows.
Solution:   Add type cast. (Yasuhiro Matsomoto)
diff --git a/src/main.c b/src/main.c
index 3b18bf1..fd96f41 100644
--- a/src/main.c
+++ b/src/main.c
@@ -2573,7 +2573,7 @@
 	     * Note that the message is shown on a Cygwin terminal (e.g.
 	     * mintty) which encoding is based on $LC_ALL or etc., not the
 	     * current codepage used by normal Win32 console programs. */
-	    tofree = s = enc_locale_env(NULL);
+	    tofree = s = (char *)enc_locale_env(NULL);
 	    if (s == NULL)
 		s = "utf-8";	/* Use "utf-8" by default. */
 	    (void)bind_textdomain_codeset(VIMPACKAGE, s);