[clangd] Unbreak mac build after 0c96a92

That commit removed the include of Features.inc from ClangdLSPServer.h,
but ClangdMain.cpp relied on this include to pull in Features.inc for
the #if at the bottom of Transport.h.

Since the include is needed in Transport.h, just add it to there
directly.

Author: nico

clang-tools-extra/clangd/Transport.h

@@ -18,6 +18,7 @@
 #ifndef LLVM_CLANG_TOOLS_EXTRA_CLANGD_TRANSPORT_H_
 #define LLVM_CLANG_TOOLS_EXTRA_CLANGD_TRANSPORT_H_
 
+#include "Features.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/JSON.h"
 #include "llvm/Support/raw_ostream.h"