Add markewaite as a conditional-buildstep maintainer

I've contributed several pull requests to the plugin, including:

* jenkinsci/conditional-buildstep-plugin#46
* jenkinsci/conditional-buildstep-plugin#47
* jenkinsci/conditional-buildstep-plugin#38
* jenkinsci/conditional-buildstep-plugin#48
* jenkinsci/conditional-buildstep-plugin#45
* jenkinsci/conditional-buildstep-plugin#44

I already have permission on the GitHub repository and have merged
several pull requests.

I'd like release permissions so that the plugin can be built and a new
release can be included in the plugin bill of materials.  Plugin BOM
pull request is:

* jenkinsci/bom#2224

permissions/plugin-conditional-buildstep.yml

@@ -12,6 +12,7 @@ developers:
   - "olamy"
   - "teilo"
   - "rarabaolaza"
+  - "markewaite"
 security:
   contacts:
     jira: "cloudbees_security_members"