devel/strace: fix build on mpc85xx target

builbot faillog:
https://downloads.lede-project.org/snapshots/faillogs/powerpc_8540/base/strace/compile.txt

Seems that for PPC, the <linux/ptrace.h> header [from uapi] collides
with musl's <sys/user.h>, for the pt_regs struct.

Seems I tried to upstream this patch a while back:
https://lists.openwrt.org/pipermail/openwrt-devel/2016-April/041093.html



It seems to work on current LEDE trunk too.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>