[tools] Allow struct types in definitions

Allow struct type names in definitions, which result in struct buffer
pointers in generated code.

scripts/test_parse_definitions.py

@@ -0,0 +1,10 @@
+import sys
+from pathlib import Path
+from definitions.context import Context
+
+root = Path(sys.argv[0]).parent.parent
+defs_path = (root / "definitions").resolve()
+print(f"Definitions: {defs_path}")
+ctx = Context(str(defs_path))
+ctx.parse("syscalls.def")
+