Enable docs on in the x86_64-unknown-linux-musl manifest

Add the rust-docs component to toolchain x86_64-unknown-linux-musl, which allows
people using rustup on their musl-based linux distribution to download the
rust-docs.

Generating and uploading the docs was enabled in b5d143b.

src/tools/build-manifest/src/main.rs

@@ -156,6 +156,7 @@ static DOCS_TARGETS: &[&str] = &[
     "x86_64-pc-windows-gnu",
     "x86_64-pc-windows-msvc",
     "x86_64-unknown-linux-gnu",
+    "x86_64-unknown-linux-musl",
 ];
 
 static MINGW: &[&str] = &["i686-pc-windows-gnu", "x86_64-pc-windows-gnu"];