boards: shields: seeed_xiao_expansion_board: fix missing include

Was getting the error "parse error: expected number or parenthesized
expression" when trying to compile with the
-DSHIELD=seeed_xiao_expansion_board flag. It looks like INPUT_KEY_0
was missing the necessary include file so I added it in.

Signed-off-by: Graham Harrison <graham.harrison@axcendcorp.com>