Add platform detection and define LINUX constant early in carla-plugin

Imported the platform module and defined the LINUX constant at the top of the script, before any usage.  
This ensures the LINUX flag is correctly set before it is referenced later in the code, preventing potential runtime errors.

source/frontend/carla-plugin

@@ -5,6 +5,9 @@
 # ------------------------------------------------------------------------------------------------------------
 # Imports (Global)
 
+import platform
+LINUX = platform.system() == "Linux"
+
 from qt_compat import qt_config
 
 if qt_config == 5: