CMake: change xcpp linkage scope to PRIVATE

Previously the linkage was PUBLIC, which caused xcpp to link to the
already existing libxeus-clang-repl.so instead of the just built one.

Signed-off-by: Pavel Kalugin <pavel@pavelthebest.me>

Author: p4vook

CMakeLists.txt

@@ -214,7 +214,7 @@ set_target_properties(xcpp PROPERTIES
   ENABLE_EXPORTS 1
   CXX_STANDARD ${CMAKE_CXX_STANDARD}
 )
-target_link_libraries(xcpp PUBLIC xeus-clang-repl pthread Python::Python)
+target_link_libraries(xcpp PRIVATE xeus-clang-repl pthread Python::Python)
 
 #TODO: We may be need sse RPATH
 #set_target_properties(xcpp clangCppInterOp PROPERTIES