futex: do not pretend <linux/futex.h> is included

futex: do not pretend <linux/futex.h> is included

As configure.ac does not check for linux/futex.h, HAVE_LINUX_FUTEX_H is
never defined and therefore the inclusion of <linux/futex.h> guarded by
HAVE_LINUX_FUTEX_H makes no sense.

Moreover, <linux/futex.h> used to have an incorrect definition
of FUTEX_WAIT_BITSET_PRIVATE and FUTEX_WAKE_BITSET_PRIVATE:
since kernel commit v2.6.24-6320-gcd68998 where these definitions
were initially introduced and up to v2.6.31-7082-gf8d1e54 where they
were finally fixed these macros had been incorrectly defined via
FUTEX_WAIT_BITS and FUTEX_WAKE_BITS instead of FUTEX_WAIT_BITSET
and FUTEX_WAKE_BITSET, and these incorrect definitions made their way
into some distributions still in use.

* futex.c [HAVE_LINUX_FUTEX_H]: Remove.