Add -Werror by default again

It was removed by mistake.

core/erlc.mk

@@ -5,7 +5,7 @@
 
 # Configuration.
 
-ERLC_OPTS ?= +debug_info +warn_export_vars +warn_shadow_vars \
+ERLC_OPTS ?= -Werror +debug_info +warn_export_vars +warn_shadow_vars \
 	+warn_obsolete_guard # +bin_opt_info +warn_export_all +warn_missing_spec
 COMPILE_FIRST ?=
 COMPILE_FIRST_PATHS = $(addprefix src/,$(addsuffix .erl,$(COMPILE_FIRST)))