Don't use a variable called 'interface'

MSVC has #define interface struct, resulting in a nonsensical error from
the compiler about "struct: missing tag name" in a line that doesn't say
"struct".

Change-Id: I1ca703fd7920052e9d0bdf9a6936df77597ff022
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>