libsoc: fix MUSL build issue

Without <linux/ioctl.h> include compiler cannot find _IOC_SIZEBITS
macro, when using MUSL C library.

Fixes:
  http://autobuild.buildroot.net/results/c86/c86fc75212df75906222772854c283396bb880d2/



Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>