Use deterministic iteration in type_support header

The `include_directives` set is used by the nested templates to
prevent generation of duplicate include directives, but is not iterated
over by those nested templates.

In `idl__type_support.c.em` rather that iterating over the
`include_directives` set (which introduces non-determinism) to generate
the includes for top level header files, we iterate over those top level
includes as a list and seperately construct the `include_directives` set
from the list. This ensures deterministic codegen.

Signed-off-by: Harry Sarson <harry.sarson@codethink.co.uk>

rosidl_generator_c/resource/idl__type_support.c.em

@@ -16,16 +16,19 @@ include_parts = [package_name] + list(interface_path.parents[0].parts) + [
     'detail', convert_camel_case_to_lower_case_underscore(interface_path.stem)]
 include_base = '/'.join(include_parts)
 
-include_directives = {
+top_level_includes = [
   'rosidl_typesupport_interface/macros.h',
   include_base + '__type_support.h',
   include_base + '__struct.h',
-  include_base + '__functions.h'}
+  include_base + '__functions.h']
+
+include_directives = set(top_level_includes)
+
 }@
 
 #include <string.h>
 
-@[for header_file in include_directives]@
+@[for header_file in top_level_includes]@
 #include "@(header_file)"
 @[end for]@