phosphor-bios-software-update: 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-bios-software-update
/tmp/phosphor-bmc-code-mgmt/usr/local/libexec/phosphor-bmc-code-mgmt/phosphor-bios-software-update
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ib0b6da9218dc0ebd67077c8cf0e5e8bb6b64d11b
diff --git a/bios/xyz.openbmc_project.Software.BIOS.service b/bios/xyz.openbmc_project.Software.BIOS.service
index e57fd55..524acab 100644
--- a/bios/xyz.openbmc_project.Software.BIOS.service
+++ b/bios/xyz.openbmc_project.Software.BIOS.service
@@ -9,7 +9,7 @@
 Restart=always
 Type=oneshot
 RemainAfterExit=no
-ExecStart=/usr/bin/phosphor-bios-software-update
+ExecStart=/usr/libexec/phosphor-code-mgmt/phosphor-bios-software-update
 
 [Install]
 WantedBy=multi-user.target