Workaround fmtlib 8.1.1 compiler error on GCC

Bug report: fmtlib/fmt#2769

src/doc/validate.h

@@ -1,12 +1,15 @@
 #pragma once
 
+// format.h contains fmt::memory_buffer, core.h doesn't.
+// Include <fmt/format.h> before "doc.h" to avoid partial specialization errors in
+// UNITTEST builds (where doc.h includes and instantiates <fmt/core.h>) on GCC.
+#include <fmt/format.h>
+#include <fmt/compile.h>
+
 #include "validate_common.h"
 #include "doc.h"
 #include "util/copy_move.h"
 
-#include <fmt/format.h>  // format.h contains fmt::memory_buffer, core.h doesn't
-#include <fmt/compile.h>
-
 #include <cstdint>
 #include <optional>
 #include <vector>