third-party/rust: update imgui

Summary:
# Stack
* Update UI libs used by Lume

# Diff
* Update `imgui-rs` using the latest commit so we can use the latest version and fixes from the original lib C `imgui`
* Vendoring the lib failed because the README file on the lib was a symbolic link to a file that was renamed so it wasn't finding it.
* ~~The solution was to make a fork https://github.com/gianluigip/imgui-rs and fix the link to the readme.~~
  * ~~This is the PR to upstream the fix imgui-rs/imgui-rs#812
  * The fix was merged into the main repo so I updated the repo and hash.

Differential Revision: D65202461

fbshipit-source-id: 0559c5a6ac2f1d43159207064d684cf1fb69dc67

Cargo.toml

@@ -6,6 +6,8 @@ cxx = { package = "cxx", git = "https://github.com/facebookexperimental/cxx.git"
 cxx-build = { package = "cxx-build", git = "https://github.com/facebookexperimental/cxx.git", rev = "2ea39c564284a70fdc1793fd72c442433e5a2638" }
 displaydoc = { git = "https://github.com/yaahc/displaydoc", rev = "7dc6e324b1788a6b7fb9f3a1953c512923a3e9f0" }
 graphql-parser = { git = "https://github.com/graphql-rust//graphql-parser", rev = "8d76425d83c40670570cc325f57c730262f07456" }
+imgui = { git = "https://github.com/imgui-rs/imgui-rs.git", rev = "2abba18c08fb5e70c699ba7992342e7ff8573b09" }
+imgui-sys = { git = "https://github.com/imgui-rs/imgui-rs.git", rev = "2abba18c08fb5e70c699ba7992342e7ff8573b09" }
 lru-disk-cache = { git = "https://github.com/mozilla/sccache", rev = "fdbf843d174c6796d736b2b61dab0297670390f8" }
 perf-event = { version = "0.4", git = "https://github.com/krallin/perf-event.git", rev = "86224a9bc025d5d19f719542f27c8c629a08b167" }
 perf-event-open-sys = { version = "4.0", git = "https://github.com/krallin/perf-event.git", rev = "86224a9bc025d5d19f719542f27c8c629a08b167" }