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.
Merge pull request #15 from osrf/fix_srv_gen_deps

Fix output file names for .srv convert files