Drop some redundant defined(_WIN64)

When it is already defined(_WIN32).

According to Microsoft documentation:
http://msdn.microsoft.com/en-us/library/b0084kay.aspx

_WIN32 Defined for applications for Win32 and Win64. Always defined.

_WIN64 Defined for applications for Win64.

Patch suggested by Joona.

No functional change.

src/platform.h

@@ -43,7 +43,7 @@ typedef unsigned __int64 uint64_t;
 #  include <unistd.h>  // Used by sysconf(_SC_NPROCESSORS_ONLN)
 #endif
 
-#if !defined(_WIN32) && !defined(_WIN64) // Linux - Unix
+#if !defined(_WIN32) // Linux - Unix
 
 #  include <sys/time.h>
 typedef timeval sys_time_t;