Ignore build_h.jl.phony

When running `version_git.sh` to build `version_git.jl`, we encode
whether the current buildtree is modified.  Unfortunately, this often
runs in parallel with the generation of `build_h.jl`, which means that
more often than not, `build_h.jl.phony` is sitting around in the build
tree, and since `git` hasn't been told to ignore that file, we get
modified build trees a lot during `make -jN`.  We don't see this in
final builds however, because when we swing back around in `make
binary-dist`, the Makefile regenerates `version_git.jl.phony`, checks it
against `version_git.jl` and notices that `git` no longer things the
build tree is dirty.  (This being because `build_h.jl.phony` does not
exist.  Note that we never run `make binary-dist` in parallel).  Thus,
we always end up with a `version_git.jl` that identifies itself as
coming from a pristine build, and the price we paid was just that we
would always build the system image twice.  Come, global warming, come.

base/.gitignore

@@ -1,6 +1,7 @@
 /pcre_h.jl
 /errno_h.jl
 /build_h.jl
+/build_h.jl.phony
 /file_constants.jl
 /uv_constants.jl
 /version_git.jl