Update formatting library.

vendor/Fmt/test/header-only-test.cc

@@ -1,7 +1,11 @@
 // Header-only configuration test
 
 #include "fmt/core.h"
+#include "fmt/ostream.h"
+#include "gtest/gtest.h"
 
 #ifndef FMT_HEADER_ONLY
 #  error "Not in the header-only mode."
 #endif
+
+TEST(header_only_test, format) { EXPECT_EQ(fmt::format("foo"), "foo"); }