fix: add missing clean targets

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

backend/go/piper/Makefile

@@ -34,4 +34,7 @@ piper: sources/go-piper sources/go-piper/libpiper_binding.a espeak-ng-data
 package:
 	bash package.sh
 
-build: piper package
+build: piper package
+
+clean:
+	rm -f piper

backend/go/silero-vad/Makefile

@@ -44,4 +44,7 @@ silero-vad: backend-assets/lib/libonnxruntime.so.1
 package:
 	bash package.sh
 
-build: silero-vad package
+build: silero-vad package
+
+clean:
+	rm -f silero-vad