http2: use correct include for snprintf

Using the first little merge of nghttp2 into libcurl, I stumbeled on the
missing 'snprintf' in MSVCRT. Isn't this how we do it for other libcurl
files?  I.e. use 'curl_msnprintf' and not 'snprintf' directly:

lib/http2.c

@@ -23,6 +23,9 @@
 #include "curl_setup.h"
 
 #ifdef USE_NGHTTP2
+#define _MPRINTF_REPLACE
+#include <curl/mprintf.h>
+
 #include <nghttp2/nghttp2.h>
 #include "http2.h"