MIPS: ioctl.h: Fix headers_check warnings

Make ioctl.h compatible with asm-generic/ioctl.h and userspace

fix the following 'make headers_check' warning:

  usr/include/asm-mips/ioctl.h:64: extern's make no sense in userspace

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>