[libc++][CMake] Fix typo introduced in r356150

That typo broke the build when the shared library build was disabled.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@356155 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 89485d7..91924f4 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -234,7 +234,7 @@
       LINK_FLAGS    "${LIBCXX_LINK_FLAGS}"
       OUTPUT_NAME   "c++"
   )
-  cxx_set_common_defines(cxx_shared)
+  cxx_set_common_defines(cxx_static)
 
   if (LIBCXX_HERMETIC_STATIC_LIBRARY)
     append_flags_if_supported(CXX_STATIC_LIBRARY_FLAGS -fvisibility=hidden)