[PATCH] libcxx random_device for Win32

[Nico Rieck]

  #  endif
+#  // Must be defined before stdlib.h is included to enable rand_s().
+#  define _CRT_RAND_S
  #endif  // _WIN32

Since rand_s is never used in a libc++ header, I don't think it should 
be defined here.

There are also trailing whitespace in the patch. The __f_ data member is 
unused and should be removed. (It also can't hold any HANDLE if that's 
the reason you kept it around.) And I see no need to duplicate entropy().

-Nico