Both glibc and the kernel have in6_* macros definitions. Build fails
because it picks up wrong in6_* macro from the kernel header and not the
header from glibc.

Fixes build error below:
clang -I. -I./include/uapi -I../../../include/uapi
     -Wno-compare-distinct-pointer-types \
         -O2 -target bpf -emit-llvm -c test_tcpbpf_kern.c -o - |      \
llc -march=bpf -mcpu=generic -filetype=obj
     -o .../tools/testing/selftests/bpf/test_tcpbpf_kern.o
In file included from test_tcpbpf_kern.c:12:
.../netinet/in.h:101:5: error: expected identifier
    IPPROTO_HOPOPTS = 0,   /* IPv6 Hop-by-Hop options.  */
    ^
.../linux/in6.h:131:26: note: expanded from macro 'IPPROTO_HOPOPTS'
                                ^
In file included from test_tcpbpf_kern.c:12:
/usr/include/netinet/in.h:103:5: error: expected identifier
    IPPROTO_ROUTING = 43,  /* IPv6 routing header.  */
    ^
.../linux/in6.h:132:26: note: expanded from macro 'IPPROTO_ROUTING'
                                ^
In file included from test_tcpbpf_kern.c:12:
.../netinet/in.h:105:5: error: expected identifier
    IPPROTO_FRAGMENT = 44, /* IPv6 fragmentation header.  */
    ^

Since both glibc and the kernel have in6_* macros definitions, use the
one from glibc.  Kernel headers will check for previous libc definitions
by including include/linux/libc-compat.h.
Reported-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Tested-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>