Print the error message on stderr in a python2 compatible way

Change-Id: Iaea1510b02a2df924ef9777d107d22c45cb37d12
diff --git a/apps/grgsm_decode b/apps/grgsm_decode
index d754fc7..000440f 100755
--- a/apps/grgsm_decode
+++ b/apps/grgsm_decode
@@ -369,7 +369,7 @@
         parser.error("Invalid A5 version\n")
 
     if options.cfile and (options.fc is None and options.arfcn is None):
-        print("You haven't provided a frequency or an ARFCN - working without automatic frequency offset correction.\n", file=sys.stderr)
+        sys.stderr.write("You haven't provided a frequency or an ARFCN - working without automatic frequency offset correction.\n")
 
     # handle frequency / arfcn input
     arfcn = None