tools: ynl-gen-c: don't require -o argument

Without -o the tool currently crashes, but it's not marked
as required. The only thing we can't do without it is to
generate the correct #include for user source files, but
we can put a placeholder instead.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20241206113100.89d35bf124d6.I9228fb704e6d5c9d8e046ef15025a47a48439c1e@changeid
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -2761,7 +2761,10 @@ def main():
         cw.p('#define ' + hdr_prot)
         cw.nl()
 
-    hdr_file=os.path.basename(args.out_file[:-2]) + ".h"
+    if args.out_file:
+        hdr_file = os.path.basename(args.out_file[:-2]) + ".h"
+    else:
+        hdr_file = "generated_header_file.h"
 
     if args.mode == 'kernel':
         cw.p('#include <net/netlink.h>')