phosphor-version-software-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-version-software-manager
/tmp/phosphor-bmc-code-mgmt/usr/local/libexec/phosphor-bmc-code-mgmt/phosphor-version-software-manager
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I89254e142fdaa4eff55ded4d052eae0399c2306b
diff --git a/bmc/xyz.openbmc_project.Software.Version.service.in b/bmc/xyz.openbmc_project.Software.Version.service.in
index 9651ee2..36b21db 100644
--- a/bmc/xyz.openbmc_project.Software.Version.service.in
+++ b/bmc/xyz.openbmc_project.Software.Version.service.in
@@ -3,7 +3,7 @@
 
 [Service]
 ExecStartPre=/bin/sh -c 'mkdir -p /tmp/images'
-ExecStart=/usr/bin/phosphor-version-software-manager
+ExecStart=/usr/libexec/phosphor-code-mgmt/phosphor-version-software-manager
 Restart=always
 Type=dbus
 BusName={BUSNAME}