Remove remaining dependencies on allocation.h from a few platform files

src/platform-posix.cc

Index: src/platform-posix.cc
diff --git a/src/platform-posix.cc b/src/platform-posix.cc
index 928edb86a5dc1c397f1c2bc79747dd7a71ef44a7..bc9d8e2fd419296037a9d7ee809e5225011ef6e4 100644
--- a/src/platform-posix.cc
+++ b/src/platform-posix.cc
@@ -492,7 +492,7 @@ void OS::StrNCpy(char* dest, int length, const char* src, size_t n) {
 // POSIX thread support.
 //
 
-class Thread::PlatformData : public Malloced {
+class Thread::PlatformData {
  public:
   PlatformData() : thread_(kNoThread) {}
   pthread_t thread_;  // Thread handle for pthread.