ynl: include uapi header after all dependencies

Essentially reverse the order of headers for userspace generated files.

Before (make -C tools/net/ynl/; cat tools/net/ynl/ethtool-user.h):
  #include <linux/ethtool_netlink_generated.h>
  #include <linux/ethtool.h>
  #include <linux/ethtool.h>
  #include <linux/ethtool.h>

After:
  #include <linux/ethtool.h>
  #include <linux/ethtool_netlink_generated.h>

While at it, make sure we track which headers we've already included
and include the headers only once.

Signed-off-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241204155549.641348-6-sdf@fomichev.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/ynl-gen-c.py

@@ -2782,12 +2782,17 @@ def main():
         else:
             cw.p(f'#include "{hdr_file}"')
             cw.p('#include "ynl.h"')
-        headers = [parsed.uapi_header]
+        headers = []
     for definition in parsed['definitions']:
         if 'header' in definition:
             headers.append(definition['header'])
+    if args.mode == 'user':
+        headers.append(parsed.uapi_header)
+    seen_header = []
     for one in headers:
-        cw.p(f"#include <{one}>")
+        if one not in seen_header:
+            cw.p(f"#include <{one}>")
+            seen_header.append(one)
     cw.nl()
 
     if args.mode == "user":