phosphor-download-manager: move to libexec

Non-user executables are suppose to be installed into libexec[1].

[1]: https://github.com/openbmc/docs/blob/master/anti-patterns.md#incorrect-placement-of-executables-in-sbin-usrsbin-or-bin-usrbin

Tested:

```
$ meson install -C builddir --destdir /tmp/phosphor-bmc-code-mgmt
$ find /tmp/phosphor-bmc-code-mgmt/ -name phosphor-download-manager
/tmp/phosphor-bmc-code-mgmt/usr/local/libexec/phosphor-bmc-code-mgmt/phosphor-download-manager
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If56d9639c20b00c8249d086af82bb62fb13ded66
diff --git a/bmc/xyz.openbmc_project.Software.Download.service.in b/bmc/xyz.openbmc_project.Software.Download.service.in
index 404691e..e6fa07f 100644
--- a/bmc/xyz.openbmc_project.Software.Download.service.in
+++ b/bmc/xyz.openbmc_project.Software.Download.service.in
@@ -2,7 +2,7 @@
 Description=Phosphor Download Manager
 
 [Service]
-ExecStart=/usr/bin/phosphor-download-manager
+ExecStart=/usr/libexec/phosphor-code-mgmt/phosphor-download-manager
 Restart=always
 Type=dbus
 BusName={BUSNAME}