trivial: add space in 'bool"package"' construct in Config.in

In early buildroot, it apparently was customary to have following style in
Config.in files:
    bool"expat"

Nowadays, only two packages remain with this style: diffutils and expat.
This trivial patch lines them up with the style:
    bool "expat"

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>