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/meson.build b/bmc/side-switch/meson.build
index 4eda07a..e04a456 100644
--- a/bmc/side-switch/meson.build
+++ b/bmc/side-switch/meson.build
@@ -6,7 +6,7 @@
     include_directories: ['..'],
     dependencies: [phosphor_logging_dep],
     install: true,
-    install_dir: get_option('bindir'),
+    install_dir: get_option('libexecdir') / 'phosphor-code-mgmt',
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(