selftests: txtimestamp: fix compilation issue

PACKET_TX_TIMESTAMP is defined in if_packet.h but it is not included in
test. Include it instead of <netpacket/packet.h> otherwise the error of
redefinition arrives.
Also fix the compiler warning about ambiguous control flow by adding
explicit braces.

Fixes: 8fe2f761cae9 ("net-timestamp: expand documentation")
Suggested-by: Willem de Bruijn <[e-mail]>
Signed-off-by: Vadim Fedorenko <[e-mail]>
Acked-by: Willem de Bruijn <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/txtimestamp.c

···
 #include <inttypes.h>
 #include <linux/errqueue.h>
 #include <linux/if_ether.h>
+#include <linux/if_packet.h>
 #include <linux/ipv6.h>
 #include <linux/net_tstamp.h>
 #include <netdb.h>
···
 #include <netinet/ip.h>
 #include <netinet/udp.h>
 #include <netinet/tcp.h>
-#include <netpacket/packet.h>
 #include <poll.h>
 #include <stdarg.h>
 #include <stdbool.h>
···
 	total_len = cfg_payload_len;
 	if (cfg_use_pf_packet || cfg_proto == SOCK_RAW) {
 		total_len += sizeof(struct udphdr);
-		if (cfg_use_pf_packet || cfg_ipproto == IPPROTO_RAW)
+		if (cfg_use_pf_packet || cfg_ipproto == IPPROTO_RAW) {
 			if (family == PF_INET)
 				total_len += sizeof(struct iphdr);
 			else
 				total_len += sizeof(struct ipv6hdr);
-
+		}
 		/* special case, only rawv6_sendmsg:
 		 * pass proto in sin6_port if not connected
 		 * also see ANK comment in net/ipv4/raw.c