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

Remove a redundant include of '<linux/icmp.h>' which is already provided in
'lwt_helpers.h'. This avoids errors seen compiling for mips64el/musl-libc:

  In file included from .../arpa/inet.h:9,
                   from lwt_redirect.c:51:
  .../netinet/in.h:23:8: error: redefinition of 'struct in6_addr'
     23 | struct in6_addr {
        |        ^~~~~~~~
  In file included from .../linux/icmp.h:24,
                   from lwt_redirect.c:50:
  .../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: 43a7c3ef8a ("selftests/bpf: Add lwt_xmit tests for BPF_REDIRECT")
Signed-off-by: Tony Ambardar <tony.ambardar@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/3869dda876d5206d2f8d4dd67331c739ceb0c7f8.1721713597.git.tony.ambardar@gmail.com

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

@@ -47,7 +47,6 @@
 #include <linux/if_ether.h>
 #include <linux/if_packet.h>
 #include <linux/if_tun.h>
-#include <linux/icmp.h>
 #include <arpa/inet.h>
 #include <unistd.h>
 #include <errno.h>