Avoid parallel invocation of git for ioemu-remote.

Avoid parallel invocation of git for ioemu-remote.

The stubdom and tools directories both run `make ioemu-dir-find' in
tools.  In a parallel build, both these invocations can run
concurrently because we're doing recursive make.

This change fixes this problem by adding a suitable dependencies in
the top-level Makefile for the recursion into tools/ and stubdom/,
ensuring that the git fetch happens once, first.

The bug was introduced in 18472/18474.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>