nfc: uapi: use kernel size_t to fix user-space builds

Fix user-space builds if it includes /usr/include/linux/nfc.h before
some of other headers:

  /usr/include/linux/nfc.h:281:9: error: unknown type name ‘size_t’
    281 |         size_t service_name_len;
        |         ^~~~~~

Fixes: d646960f ("NFC: Initial LLCP support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/nfc.h

@@ -278,7 +278,7 @@ struct sockaddr_nfc_llcp {
 	__u8 dsap; /* Destination SAP, if known */
 	__u8 ssap; /* Source SAP to be bound to */
 	char service_name[NFC_LLCP_MAX_SERVICE_NAME]; /* Service name URI */;
-	size_t service_name_len;
+	__kernel_size_t service_name_len;
 };
 
 /* NFC socket protocols */