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

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 <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/e986ba2d7edccd254b54f7cd049b98f10bafa8c3.1721713597.git.tony.ambardar@gmail.com