THRIFT-2082 Executing "gmake clean" is broken

Patch: Pavlin Radoslavov
diff --git a/lib/erl/Makefile.am b/lib/erl/Makefile.am
index 089cca5..6550b52 100644
--- a/lib/erl/Makefile.am
+++ b/lib/erl/Makefile.am
@@ -48,7 +48,7 @@
 
 clean:
 	./rebar clean
-	rm .generated
+	rm -f .generated
 	rm -f test/secondService_* \
 		  test/aService_* \
 		  test/serviceForExceptionWithAMap_* \