Fixed the mingw build by adding compatibility code for vsnprintf()

PiperOrigin-RevId: 448611947

upb/json_encode.c

diff --git a/upb/json_encode.c b/upb/json_encode.c
index 9668a42..9af7a13 100644
--- a/upb/json_encode.c
+++ b/upb/json_encode.c
@@ -37,6 +37,7 @@
 #include <string.h>
 
 #include "upb/decode.h"
+#include "upb/internal/vsnprintf_compat.h"
 #include "upb/reflection.h"
 #include "upb/upb_internal.h"
 
@@ -112,7 +113,7 @@
   va_list args;
 
   va_start(args, fmt);
-  n = vsnprintf(e->ptr, have, fmt, args);
+  n = _upb_vsnprintf(e->ptr, have, fmt, args);
   va_end(args);
 
   if (UPB_LIKELY(have > n)) {