Use format macro to dump proof expiry

Author: Doy-lee

tests/test_pro_backend.cpp

@@ -4,6 +4,7 @@
 #include <sodium.h>
 
 #include <catch2/catch_test_macros.hpp>
+#include <cinttypes>
 #include <nlohmann/json.hpp>
 #include <session/config/pro.hpp>
 #include <session/pro_backend.hpp>
@@ -31,7 +32,7 @@ static bool string8_equals(string8 s8, std::string_view str) {
     fprintf(stderr,
             "proof.rotating_pubkey: %s\n",
             oxenc::to_hex(proof.rotating_pubkey.data).c_str());
-    fprintf(stderr, "proof.expiry_unix_ts_ms: %zu\n", proof.expiry_unix_ts_ms);
+    fprintf(stderr, "proof.expiry_unix_ts_ms: %" PRIu64 "\n", proof.expiry_unix_ts_ms);
     fprintf(stderr, "proof.sig: %s\n", oxenc::to_hex(proof.sig.data).c_str());
 }