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

PiperOrigin-RevId: 448611947

BUILD

diff --git a/BUILD b/BUILD
index ef032b9..c4d796f 100644
--- a/BUILD
+++ b/BUILD
@@ -83,6 +83,9 @@
 
 cc_library(
     name = "port",
+    hdrs = [
+        "upb/internal/vsnprintf_compat.h",
+    ],
     copts = UPB_DEFAULT_COPTS,
     textual_hdrs = [
         "upb/port_def.inc",