Fix output file names for .srv convert files

Create the correct OUTPUT files that should be expected from running the idl converter generator on a .srv file.  Previously, from `Foo.srv`, it was expecting to generate `Foo_convert.h` instead of `FooRequest_convert.h` and `FooResponse_convert.h`, which meant that the converter generator was
running again on every build.

@dirk-thomas : for your consideration