Make --disable-trace a build-time failure instead of link-time one.

Reported-by: Toralf Förster
Bug: https://bugs.gentoo.org/624570
diff --git a/include/trace.h b/include/trace.h
index f05dd0e..398e939 100644
--- a/include/trace.h
+++ b/include/trace.h
@@ -29,2 +29,4 @@
 
+#ifdef PR_USE_TRACE
+
 #define PR_TRACE_DEFAULT_CHANNEL	"DEFAULT"
@@ -64,2 +66,4 @@ int pr_trace_vmsg(const char *, int, const char *, va_list);
 
+#endif /* PR_USE_TRACE */
+
 #endif /* PR_TRACE_H */