selftests/bpf: Fix errors compiling decap_sanity.c with musl libc

Remove a redundant include of '<linux/in6.h>', whose needed definitions are
already provided by 'test_progs.h'. This avoids errors seen compiling for
mips64el/musl-libc:

In file included from .../arpa/inet.h:9,
from ./test_progs.h:17,
from prog_tests/decap_sanity.c:9:
.../netinet/in.h:23:8: error: redefinition of 'struct in6_addr'
23 | struct in6_addr {
| ^~~~~~~~
In file included from decap_sanity.c:7:
.../linux/in6.h:33:8: note: originally defined here
33 | struct in6_addr {
| ^~~~~~~~
.../netinet/in.h:34:8: error: redefinition of 'struct sockaddr_in6'
34 | struct sockaddr_in6 {
| ^~~~~~~~~~~~
.../linux/in6.h:50:8: note: originally defined here
50 | struct sockaddr_in6 {
| ^~~~~~~~~~~~
.../netinet/in.h:42:8: error: redefinition of 'struct ipv6_mreq'
42 | struct ipv6_mreq {
| ^~~~~~~~~
.../linux/in6.h:60:8: note: originally defined here
60 | struct ipv6_mreq {
| ^~~~~~~~~

Fixes: 70a00e2f1dba ("selftests/bpf: Test bpf_skb_adjust_room on CHECKSUM_PARTIAL")
Signed-off-by: Tony Ambardar <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/e986b[e-mail]

tools/testing/selftests/bpf/prog_tests/decap_sanity.c

···
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <net/if.h>
-#include <linux/in6.h>
 
 #include "test_progs.h"
 #include "network_helpers.h"