servo: Merge #19732 - Update Gentoo Linux setup to include clang (from gootorov:master); r=jdm

<!-- 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.
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- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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

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Source-Repo: https://github.com/servo/servo
Source-Revision: d0300ffd67c2278f6c9ce8cac23923f13641abf3

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 3ecedd3f8ddafe7a01d5e24c7dc2fa452480d69a

servo/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)