net: dccp: Add __printf() markup to fix -Wsuggest-attribute=format

net/dccp/ccid.c: In function ‘ccid_kmem_cache_create’:
net/dccp/ccid.c:85:2: warning: function ‘ccid_kmem_cache_create’ might be a candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
   85 |  vsnprintf(slab_name_fmt, CCID_SLAB_NAME_LENGTH, fmt, args);

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>