[libcxx] [test] Include <cassert> for assert().

This fixes compiler errors with MSVC's STL.

llvm-svn: 346911
diff --git a/libcxx/test/std/containers/associative/set/merge.pass.cpp b/libcxx/test/std/containers/associative/set/merge.pass.cpp
index 0896596..bbae22c 100644
--- a/libcxx/test/std/containers/associative/set/merge.pass.cpp
+++ b/libcxx/test/std/containers/associative/set/merge.pass.cpp
@@ -23,6 +23,7 @@
 //   void merge(multiset<key_type, C2, allocator_type>&& source);
 
 #include <set>
+#include <cassert>
 #include "test_macros.h"
 #include "Counter.h"