fscrypt: include <linux/ioctl.h> in UAPI header

fscrypt: include <linux/ioctl.h> in UAPI header

<linux/fscrypt.h> defines ioctl numbers using the macros like _IOWR()
which are defined in <linux/ioctl.h>, so <linux/ioctl.h> should be
included as a prerequisite, like it is in many other kernel headers.

In practice this doesn't really matter since anyone referencing these
ioctl numbers will almost certainly include <sys/ioctl.h> too in order
to actually call ioctl().  But we might as well fix this.

Link: https://lore.kernel.org/r/20191219185624.21251-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>