[lldb][test] Only link Windows libraries on Windows

ld.lld: error: unable to find library -llldbPluginProcessWindowsCommon

https://lab.llvm.org/buildbot/#/builders/96/builds/50407

Fixes 95e5839.

Author: DavidSpickett

lldb/unittests/Thread/CMakeLists.txt

@@ -1,3 +1,8 @@
+if (CMAKE_SYSTEM_NAME MATCHES "Windows")
+  list(APPEND LLDB_WINDOWS_LIBS lldbPluginProcessWindows)
+  list(APPEND LLDB_WINDOWS_LIBS lldbPluginProcessWindowsCommon)
+endif()
+
 add_lldb_unittest(ThreadTests
   ThreadTest.cpp
 
@@ -11,7 +16,6 @@ add_lldb_unittest(ThreadTests
       lldbInterpreter
       lldbBreakpoint
       lldbPluginPlatformLinux
-      lldbPluginPlatformWindows
-      lldbPluginProcessWindowsCommon
+      ${LLDB_WINDOWS_LIBS}
   )