Ensure read_file() files are considered by "gn analyze"

Just like imports, these files need to be added as build file
dependencies.

Found this from a chromium CL where analyze missed that a .info file,
read via read_file(), was changed.

Tested locally that this now results in a changed target:

{
    "files": [ "//third_party/android_deps/libs/android_arch_lifecycle_viewmodel/android_arch_lifecycle_viewmodel.info" ],
    "additional_compile_targets": [],
    "test_targets": [ "//chrome/android:chrome_public_test_apk" ]
}

Bug: None
Change-Id: Ifaca46ff87d43f4d45123cbdb67162298d6919f8
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/16580
Reviewed-by: Brett Wilson <brettw@google.com>
Commit-Queue: Brett Wilson <brettw@google.com>
diff --git a/src/gn/function_read_file.cc b/src/gn/function_read_file.cc
index acb5444..95fae08 100644
--- a/src/gn/function_read_file.cc
+++ b/src/gn/function_read_file.cc
@@ -57,6 +57,7 @@
       args[0], err, scope->settings()->build_settings()->root_path_utf8());
   if (err->has_error())
     return Value();
+  scope->AddBuildDependencyFile(source_file);
   base::FilePath file_path =
       scope->settings()->build_settings()->GetFullPath(source_file);