Userland: Properly populate GENERATED_SOURCES

We previously put the generated headers in SOURCES, which did not mark
them as GENERATED (and did not produce a proper dependency).
This commit moves all generated headers into GENERATED_SOURCES, and
removes useless header SOURCES.

Userland/Services/WebContent/CMakeLists.txt

@@ -12,12 +12,15 @@ set(SOURCES
     ConsoleGlobalObject.cpp
     ImageCodecPluginSerenity.cpp
     PageHost.cpp
-    WebContentClientEndpoint.h
     WebContentConsoleClient.cpp
-    WebContentServerEndpoint.h
     main.cpp
 )
 
+set(GENERATED_SOURCES
+    WebContentClientEndpoint.h
+    WebContentServerEndpoint.h
+)
+
 serenity_bin(WebContent)
 target_link_libraries(WebContent LibCore LibIPC LibGfx LibImageDecoderClient LibWebView LibWeb LibMain)
 link_with_locale_data(WebContent)