patch 8.0.1681: the format attribute fails with MinGW

Problem:    The format attribute fails with MinGW. (John Marriott)
Solution:   Don't use the format attribute with MinGW.
diff --git a/src/vim.h b/src/vim.h
index 626c0ad..4b41857 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2085,6 +2085,10 @@
 typedef struct stat stat_T;
 #endif
 
+#if defined(__GNUC__) && !defined(__MINGW32__)
+# define USE_PRINTF_FORMAT_ATTRIBUTE
+#endif
+
 typedef enum
 {
     ASSERT_EQUAL,