tools/mceinject: Fix build after 31d2d1e08a

tools/mceinject: Fix build after 31d2d1e08a

xen-mceinj is another utility which incorrectly makes use of xg_private.h  Fix
up its include path.

While fixing this Makefile, remove some trailing whitespace.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Stefano Stabellini <stefano.stabellini@citrix.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>