In order to not declare the same variable in multiple objects (which
happens when building UCIS-enabled keymap for both the ErgoDox EZ and
the ErgoDox Infinity), move the declaration to the .c file, and keep
only an extern reference in the header.
Many thanks to @fredizzimo for spotting the error in Travis, and
suggesting the fix.
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>