[CMake] Bump missing entry for an added .apinotes file to an error.

It was a warning because I didn't want to break anything when I first
added the check, but we've been warning-free for a while now. Let's
keep it that way.

Author: jrose-apple

apinotes/CMakeLists.txt

@@ -42,6 +42,6 @@ foreach(file ${SWIFT_API_NOTES_INPUT_FILES})
   get_filename_component(name "${file}" NAME_WE)
   list(FIND SWIFT_API_NOTES_INPUTS "${name}" name_index)
   if(name_index EQUAL -1)
-    message(WARNING "Found apinotes for ${name}; please add to CMakeLists.txt")
+    message(SEND_ERROR "Found apinotes for ${name}; please add to CMakeLists.txt")
   endif()
 endforeach()