This avoids the Makefile target colliding with the submodule name.
If both have the same name, the target would not be invoked.
This is because make would find a file/folder ninja in the build
subdirectory, and therefore belives that the dependencies for
the target "ninja" are fulfilled.

Change-Id: I7ef1974e06d9cd7a6023d0d17170e0bf730c4b87
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>