This patch modifies the code generator to only generate C-language
`#include <imported_schema.h>` lines for imports that are actually used.
Imagine this transitive import scenario: schema A imports a definition
from schema B. However, the definition imported from schema B was
itself imported from schema C.
In this case, the code generated for schema A need not include the
header file for schema B, as it only needs definitions from schema C.
Signed-off-by: Curt Brune <curt@enfabrica.net>
1772643adb