patch 7.4.1334

Problem:    Many compiler warnings with MingW.
Solution:   Add type casts. (Yasuhiro Matsumoto)

src/main.c

@@ -3940,7 +3940,7 @@ build_drop_cmd(
     }
     cdp = vim_strsave_escaped_ext(cwd,
 #ifdef BACKSLASH_IN_FILENAME
-		    "",  /* rem_backslash() will tell what chars to escape */
+		    (char_u *)"",  /* rem_backslash() will tell what chars to escape */
 #else
 		    PATH_ESC_CHARS,
 #endif