Remove deprecation warning

As a workaround for some devices, we need to prepare the main looper.
The method is now deprecated, but we still want to call it.

server/src/main/java/com/genymobile/scrcpy/Workarounds.java

@@ -16,6 +16,7 @@ public final class Workarounds {
         // not instantiable
     }
 
+    @SuppressWarnings("deprecation")
     public static void prepareMainLooper() {
         // Some devices internally create a Handler when creating an input Surface, causing an exception:
         //   "Can't create handler inside thread that has not called Looper.prepare()"