Add explicit platform dependency

This dependency is technically redundant as long as we also have the
dependency on com.intellij.modules.lang. However, specifying the
dependency explicitly may avoid confusion, especially because the
documentation were sometimes written as if you have to specify this
dependency if the plugin shall work on other IDEs as IntelliJ.

src/main/resources/META-INF/plugin.xml

@@ -4,6 +4,7 @@
     <name>NixIDEA</name>
     <vendor>NixOS</vendor>
 
+    <depends>com.intellij.modules.platform</depends>
     <depends>com.intellij.modules.lang</depends>
     <depends optional="true" config-file="nix-idea-ultimate.xml">com.intellij.modules.ultimate</depends>