random: include __config before building

We need to include __config to ensure that we know what random
implementation is being used.  Fixes compilation for Windows.

llvm-svn: 290775
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 4a12eab82829e762a9a424126bf21429f5ba1b12
diff --git a/src/random.cpp b/src/random.cpp
index 4ab424e..eb2510a 100644
--- a/src/random.cpp
+++ b/src/random.cpp
@@ -7,6 +7,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <__config>
+
 #if defined(_LIBCPP_USING_WIN32_RANDOM)
 // Must be defined before including stdlib.h to enable rand_s().
 #define _CRT_RAND_S