.gitignore: ignore docs/_build

That is where the documentation gets built to.

I'm pretty much clueless about SVN; I would appreciate if someone more
knowledgeable about SVN made the equivalent change.

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@176417 91177308-0d34-0410-b5e6-96231b3b80d8

.gitignore

@@ -29,3 +29,4 @@ cscope.out
 #==============================================================================#
 # Directories to ignore (do not add trailing '/'s, they skip symlinks).
 #==============================================================================#
+docs/_build