[clangd] Add dependency on remote index service proto

It requires Index.proto to be built first. Failed builds:
https://github.com/clangd/clangd/runs/1305985916

Author: kirillbobyrev

clang-tools-extra/clangd/index/remote/CMakeLists.txt

@@ -1,6 +1,7 @@
 if (CLANGD_ENABLE_REMOTE)
   generate_protos(RemoteIndexServiceProto "Service.proto" GRPC)
   generate_protos(RemoteIndexProto "Index.proto")
+  add_dependencies(RemoteIndexServiceProto RemoteIndexProto)
   include_directories(${CMAKE_CURRENT_BINARY_DIR})
   include_directories(${CMAKE_CURRENT_SOURCE_DIR}/../../)