selftests/bpf: Fix errors compiling crypto_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/crypto_sanity.c:10:
  .../netinet/in.h:23:8: error: redefinition of 'struct in6_addr'
     23 | struct in6_addr {
        |        ^~~~~~~~
  In file included from crypto_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: 91541ab1 ("selftests: bpf: crypto skcipher algo selftests")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Link: https://lore.kernel.org/bpf/911293968f424ad7b462d8805aeb3baee8f4985b.1721713597.git.tony.ambardar@gmail.com

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

@@ -4,7 +4,6 @@
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <net/if.h>
-#include <linux/in6.h>
 #include <linux/if_alg.h>
 
 #include "test_progs.h"