MINOR: quic: Add a typedef for unsigned long long

As we manipulate very uint64_t variables which must be cast to
unsigned long long to be printed, let's add this useful type definition.
diff --git a/include/haproxy/xprt_quic-t.h b/include/haproxy/xprt_quic-t.h
index df0125e..1bbae96 100644
--- a/include/haproxy/xprt_quic-t.h
+++ b/include/haproxy/xprt_quic-t.h
@@ -41,6 +41,8 @@
 #include <import/eb64tree.h>
 #include <import/ebmbtree.h>
 
+typedef unsigned long long ull;
+
 #define QUIC_PROTOCOL_VERSION_DRAFT_28   0xff00001c /* draft-28 */
 #define QUIC_PROTOCOL_VERSION_DRAFT_29   0xff00001d /* draft-29 */
 #define QUIC_PROTOCOL_VERSION_1          0x00000001 /* V1 */