Fix generated code for interface names not starting with qt_ or wl_

The generated code for an interface without a qt_ or wl_ prefix was
broken, and could not compile.
This commit needs a 'make clean' to let qtwaylandscanner run again.

Change-Id: Id9c7e3f87385f83eeffd1e8718c854a405f411f0
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>