[cmake] Fix build of attribute plugin example on Windows

Seems '${cmake_2_8_12_PRIVATE}' was removed a long time ago, so it should
be just PRIVATE keyword here.

Reviewed By: john.brawn

Differential Revision: https://reviews.llvm.org/D86091

(cherry picked from commit 04ea680)

clang/examples/Attribute/CMakeLists.txt

@@ -1,7 +1,7 @@
 add_llvm_library(Attribute MODULE Attribute.cpp PLUGIN_TOOL clang)
 
 if(LLVM_ENABLE_PLUGINS AND (WIN32 OR CYGWIN))
-  target_link_libraries(Attribute ${cmake_2_8_12_PRIVATE}
+  target_link_libraries(Attribute PRIVATE
     clangAST
     clangBasic
     clangFrontend