minor: fix mismatch of struct/class forward decl of 'Source'

Fixes the following warning and the indicate potential issue:

src/libstore/worker-protocol.hh:66:1: warning: class 'Source' was previously declared as a struct; this is valid, but may result in linker errors
under the Microsoft C++ ABI [-Wmismatched-tags]

(cherry picked from commit 6e1bb04870b1b723282d32182af286646f13bf3c)