fix time_t format specifier

Index: lib/mu-store.cc
--- lib/mu-store.cc.orig
+++ lib/mu-store.cc
@@ -603,7 +603,7 @@ Store::set_dirstamp(const std::string& path, time_t ts
 {
 	std::array<char, 2 * sizeof(tstamp) + 1> data{};
 	const auto len = static_cast<size_t>(
-	    g_snprintf(data.data(), data.size(), "%zx", tstamp));
+	    g_snprintf(data.data(), data.size(), "%lld", (long long)tstamp));
 
 	set_metadata(path, std::string{data.data(), len});
 }