BUILD: do not disable -Wformat-truncation anymore

There were very few entries to fix and this warning, while often
wrong, can actually spot future issues. If it can help developers
adjust their code in the future to make it more robust, it's not
necessarily that bad. Let's re-enable it and see how it goes.
diff --git a/Makefile b/Makefile
index fd8edc5..edbdbe3 100644
--- a/Makefile
+++ b/Makefile
@@ -182,7 +182,6 @@
 # to be sure we get the intended behavior.
 SPEC_CFLAGS := -fno-strict-aliasing -Wdeclaration-after-statement
 SPEC_CFLAGS += $(call cc-opt-alt,-fwrapv,$(call cc-opt,-fno-strict-overflow))
-SPEC_CFLAGS += $(call cc-nowarn,format-truncation)
 SPEC_CFLAGS += $(call cc-nowarn,address-of-packed-member)
 SPEC_CFLAGS += $(call cc-nowarn,unused-label)
 SPEC_CFLAGS += $(call cc-nowarn,sign-compare)