gcn: Fix a warning

gcn: Fix a warning

I see
../../gcc/config/gcn/gcn.cc: In function ‘void gcn_hsa_declare_function_name(FILE*, const char*, tree)’:
../../gcc/config/gcn/gcn.cc:6568:67: warning: unused parameter ‘decl’ [-Wunused-parameter]
 6568 | gcn_hsa_declare_function_name (FILE *file, const char *name, tree decl)
      |                                                              ~~~~~^~~~
warning presumably since r14-6945-gc659dd8bfb55e02a1b97407c1c28f7a0e8f7f09b
Previously, the argument was anonymous, but now it is passed to a macro
which ignores it, so I think we should go with ATTRIBUTE_UNUSED.

2024-01-23  Jakub Jelinek  <jakub@redhat.com>

	* config/gcn/gcn.cc (gcn_hsa_declare_function_name): Add
	ATTRIBUTE_UNUSED to decl.