selftests/bpf: fix broken build due to types.h

Commit 0a5539f6 ("bpf: Provide a linux/types.h override
for bpf selftests.") caused a build failure for tools/testing/selftest/bpf
because of some missing types:
    $ make -C tools/testing/selftests/bpf/
    ...
    In file included from /home/yhs/work/net-next/tools/testing/selftests/bpf/test_pkt_access.c:8:
    ../../../include/uapi/linux/bpf.h:170:3: error: unknown type name '__aligned_u64'
                    __aligned_u64   key;
    ...
    /usr/include/linux/swab.h:160:8: error: unknown type name '__always_inline'
    static __always_inline __u16 __swab16p(const __u16 *p)
    ...
The type __aligned_u64 is defined in linux:include/uapi/linux/types.h.

The fix is to copy missing type definition into
tools/testing/selftests/bpf/include/uapi/linux/types.h.
Adding additional include "string.h" resolves __always_inline issue.

Fixes: 0a5539f6 ("bpf: Provide a linux/types.h override for bpf selftests.")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>