[libc] Migrate to using LIBC_NAMESPACE_DECL for namespace declaration (#98075)

This is a part of #97655.

libc/fuzzing/__support/hashtable_fuzz.cpp

@@ -12,8 +12,9 @@
 #include "include/llvm-libc-types/ENTRY.h"
 #include "src/__support/CPP/string_view.h"
 #include "src/__support/HashTable/table.h"
+#include "src/__support/macros/config.h"
 
-namespace LIBC_NAMESPACE {
+namespace LIBC_NAMESPACE_DECL {
 
 // A fuzzing payload starts with
 // - uint16_t: initial capacity for table A
@@ -179,4 +180,4 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
   return 0;
 }
 
-} // namespace LIBC_NAMESPACE
+} // namespace LIBC_NAMESPACE_DECL