bssmap_codec: fix bin_bcd2str (cought by dialyzer)
diff --git a/src/bssmap_codec.erl b/src/bssmap_codec.erl
index 61cf85e..530c62e 100644
--- a/src/bssmap_codec.erl
+++ b/src/bssmap_codec.erl
@@ -193,5 +193,5 @@
 	List;
 bin_bcd2str(BcdBin, List) ->
 	<<Nibble:4, Remain/bitstring>> = BcdBin,
-	Char = "0" + Nibble,
+	Char = $0 + Nibble,
 	bin_bcd2str(Remain, List ++ [Char]).