cppgc: Fix "#if" directive in logging.h

The macro may be defined but the value to it may not be assigned. The
compiler can complain about (reproduced when compiling chromium on
Windows).

Bug: chromium:1249550
Change-Id: I684997ccda7122f56c01391e7a1339fadf6589e6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3297888Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78040}

include/cppgc/internal/logging.h

@@ -20,18 +20,18 @@ FatalImpl(const char*, const SourceLocation& = SourceLocation::Current());
 template <typename>
 struct EatParams {};
 
-#if DEBUG
+#if defined(DEBUG)
 #define CPPGC_DCHECK_MSG(condition, message)  \
   do {                                        \
     if (V8_UNLIKELY(!(condition))) {          \
       ::cppgc::internal::DCheckImpl(message); \
     }                                         \
   } while (false)
-#else
+#else  // !defined(DEBUG)
 #define CPPGC_DCHECK_MSG(condition, message)                \
   (static_cast<void>(::cppgc::internal::EatParams<decltype( \
                          static_cast<void>(condition), message)>{}))
-#endif
+#endif  // !defined(DEBUG)
 
 #define CPPGC_DCHECK(condition) CPPGC_DCHECK_MSG(condition, #condition)