plugins: Make CMake fail if obs-websocket is not found

plugins/CMakeLists.txt

@@ -82,6 +82,12 @@ elseif(OS_OPENBSD)
   obs_status(STATUS "obs-vst plugin not available.")
 endif()
 
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/obs-websocket/CMakeLists.txt)
+  add_subdirectory(obs-websocket)
+else()
+  obs_status(FATAL_ERROR "obs-websocket submodule not available.")
+endif()
+
 add_subdirectory(image-source)
 add_subdirectory(obs-x264)
 add_subdirectory(obs-libfdk)
@@ -92,4 +98,3 @@ add_subdirectory(obs-transitions)
 add_subdirectory(rtmp-services)
 add_subdirectory(text-freetype2)
 add_subdirectory(aja)
-add_subdirectory(obs-websocket)