Remove extraneous fprintf args

Also fix build scripts to die due to such warnings.

Change-Id: Ia007744fcef8e37ba73b4f73e7c50c9fc9a1d4f5
diff --git a/test/023-many-interfaces/build b/test/023-many-interfaces/build
index 4ea52d9..ad42a2d 100644
--- a/test/023-many-interfaces/build
+++ b/test/023-many-interfaces/build
@@ -18,7 +18,7 @@
 set -e
 
 # Write out a bunch of interface source files.
-gcc -o iface-gen iface-gen.c
+gcc -Wall -Werror -o iface-gen iface-gen.c
 ./iface-gen
 
 mkdir classes