phosphor-bmc-side-switch: 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-bmc-side-switch
/tmp/phosphor-bmc-code-mgmt/usr/local/libexec/phosphor-bmc-code-mgmt/phosphor-bmc-side-switch
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ifbfbc598603e7606e4a1d2d832a467796cd11fde
diff --git a/bmc/side-switch/phosphor-bmc-side-switch.service b/bmc/side-switch/phosphor-bmc-side-switch.service
index 7e1c8dd..f613700 100644
--- a/bmc/side-switch/phosphor-bmc-side-switch.service
+++ b/bmc/side-switch/phosphor-bmc-side-switch.service
@@ -16,7 +16,7 @@
 Restart=no
 Type=oneshot
 RemainAfterExit=no
-ExecStart=/usr/bin/phosphor-bmc-side-switch
+ExecStart=/usr/libexec/phosphor-code-mgmt/phosphor-bmc-side-switch
 
 [Install]
 #WantedBy=obmc-host-startmin@0.target