fix '_lseeki64' undefined compiler warning

Fixes the following compiler warning on Windows:
`'_lseeki64' undefined; assuming extern returning int`

Author: richardlau

src/uvwasi.c

@@ -11,6 +11,7 @@
 # define SLASH_STR "/"
 # define IS_SLASH(c) ((c) == '/')
 #else
+# include <io.h>
 # define SLASH '\\'
 # define SLASH_STR "\\"
 # define IS_SLASH(c) ((c) == '/' || (c) == '\\')