Base DSP functions are not exported

[martinholters]

Ref. JuliaLang/julia#21956 (comment).

E.g.:

julia> VERSION
v"0.7.0-DEV.1776"

julia> Pkg.status("DSP")
 - DSP                           0.3.2

julia> :xcorr in names(DSP)
false

julia> :xcorr in names(DSP, true)
true