Fix IWYU in fxcrt headers.

core/include/fxcrt/fx_memory.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef _FX_MEMORY_H_
 #define _FX_MEMORY_H_
-#ifndef _FX_SYSTEM_H_
+
 #include "fx_system.h"
-#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 #define FX_Alloc(type, size)                                            (type*)calloc(size, sizeof(type))
 #define FX_Realloc(type, ptr, size)                                     (type*)realloc(ptr, sizeof(type) * (size))
 #define FX_AllocNL(type, size)                                          FX_Alloc(type, size)
 #define FX_ReallocNL(type, ptr, size)                           FX_Realloc(type, ptr, size)
 #define FX_Free(ptr)                                                            free(ptr)
 void*   FXMEM_DefaultAlloc(size_t byte_size, int flags);
 void*   FXMEM_DefaultRealloc(void* pointer, size_t new_size, int flags);
@@ skipping 103 matching lines @@
 
     void        FreeAll();
 private:
 
     size_t      m_TrunkSize;
 
     void*       m_pFirstTrunk;
 };
 #endif
 #endif