web_service: Move web_result.h into web_service

This is the only place it's actively used. It's also more appropriate
for web-related structures to be within the web service target.
Especially given this one doesn't rely on anything in the common
library.

src/web_service/CMakeLists.txt

@@ -5,6 +5,7 @@ add_library(web_service STATIC
     verify_login.h
     web_backend.cpp
     web_backend.h
+    web_result.h
 )
 
 create_target_directory_groups(web_service)