core/CMakeLists: Use target_compile_definitions instead of add_definitions for specifying ENABLE_WEB_SERVICE

Avoids introducing the definition to the whole directory space and
localizes it to being added to the library that needs it.

src/core/CMakeLists.txt

@@ -397,7 +397,7 @@ create_target_directory_groups(core)
 target_link_libraries(core PUBLIC common PRIVATE audio_core video_core)
 target_link_libraries(core PUBLIC Boost::boost PRIVATE fmt lz4_static mbedtls opus unicorn open_source_archives)
 if (ENABLE_WEB_SERVICE)
-    add_definitions(-DENABLE_WEB_SERVICE)
+    target_compile_definitions(core PRIVATE -DENABLE_WEB_SERVICE)
     target_link_libraries(core PUBLIC json-headers web_service)
 endif()