add /build to idea excluded dirs

.idea/riscemu.iml

@@ -7,6 +7,7 @@
       <excludeFolder url="file://$MODULE_DIR$/dist" />
       <excludeFolder url="file://$MODULE_DIR$/.mypy_cache" />
       <excludeFolder url="file://$MODULE_DIR$/riscemu.egg-info" />
+      <excludeFolder url="file://$MODULE_DIR$/build" />
     </content>
     <orderEntry type="inheritedJdk" />
     <orderEntry type="sourceFolder" forTests="false" />