Add root_gen_dir to crashpad_config

Follows up on discussion in:
https://groups.google.com/a/chromium.org/g/crashpad-dev/c/1-QgplOJnw8

Change-Id: I6def168182f5d3219db4691d4b81ea508fd08a85
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/3577803
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Joshua Peraza <jperaza@chromium.org>

BUILD.gn

@@ -17,7 +17,10 @@ import("build/test.gni")
 import("util/net/tls.gni")
 
 config("crashpad_config") {
-  include_dirs = [ "." ]
+  include_dirs = [
+    ".",
+    root_gen_dir,
+  ]
 }
 
 if (crashpad_is_in_chromium || crashpad_is_in_fuchsia) {