Makefile: call Dialyzer with -nn flag

This speeds up 'make dialyzer' for the common case of Erlang/OTP built
without --enable-native-libs.

Makefile

@@ -24,7 +24,7 @@ dialyzer: dialyzer_warnings
 	@diff -U0 dialyzer_reference dialyzer_warnings
 
 dialyzer_warnings:
-	-@dialyzer -q -n ebin -Wunmatched_returns -Werror_handling \
+	-@dialyzer -q -nn -n ebin -Wunmatched_returns -Werror_handling \
 		-Wrace_conditions > dialyzer_warnings
 
 binary: VSN = $(shell ./rebar -V)