x86/vdso: Fix the x86 vdso2c tool includes

x86/vdso: Fix the x86 vdso2c tool includes

The build-time tool arch/x86/vdso/vdso2c.c includes <linux/elf.h>,
but cannot find it, unless the build host happens to provide it.

It should be reading the uapi linux/elf.h

This build regression came along with the vdso2c changes between
v3.15 and v3.16.

Signed-off-by: Tommi Kyntola <tommi.kyntola@gmail.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/1525002.3cJ7BySVpA@musta
Link: http://lkml.kernel.org/r/efe1ec29eda830b1d0030882706f3dac99ce1f73.1427482099.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>