Auto merge of #19732 - gootorov:master, r=jdm

Update Gentoo Linux setup to include clang

<!-- Please describe your changes on the following line: -->While other Linux distributions include clang package as required in the setup section of README, Gentoo does not. This patch fixes it.
---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because it is a documentation fix

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/19732)
<!-- Reviewable:end -->

README.md

@@ -115,7 +115,7 @@ sudo pacman -S --needed base-devel git python2 python2-virtualenv python2-pip me
 sudo emerge net-misc/curl media-libs/freeglut \
     media-libs/freetype media-libs/mesa dev-util/gperf \
     dev-python/virtualenv dev-python/pip dev-libs/openssl \
-    x11-libs/libXmu media-libs/glu x11-base/xorg-server
+    x11-libs/libXmu media-libs/glu x11-base/xorg-server sys-devel/clang
 ```
 #### On Windows (MSVC)