tree-wide: several cleanups for generating symbol lists and gperf files

- pass our system include directories to make generators use our libc
  wrappers and latest kernel headers,
- include relevant headers in generated gperf file,
- use files() rather than find_program(), as the result of
  find_program() cannot be passed to 'input' of custom_target(),
- move generate-bpf-delegate-configs.py to src/core/, as it is only used
  by libcore.

tools/xml_helper.py

@@ -11,7 +11,7 @@ class CustomResolver(tree.Resolver):
         if 'ethtool-link-mode' in url:
             return self.resolve_filename('src/shared/ethtool-link-mode.xml', context)
         if 'bpf-delegate' in url:
-            return self.resolve_filename('man/bpf-delegate.xml', context)
+            return self.resolve_filename('src/core/bpf-delegate.xml', context)
 
         return None