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/meson.build b/bmc/meson.build
index 7909c58..28caf18 100644
--- a/bmc/meson.build
+++ b/bmc/meson.build
@@ -230,6 +230,7 @@
     software_common_sources,
     dependencies: [deps, ssl],
     install: true,
+    install_dir: get_option('libexecdir') / 'phosphor-code-mgmt',
 )
 
 install_data(