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)

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstore/worker-protocol.hh b/src/libstore/worker-protocol.hh
index a03edd10f..6762b609d 100644
--- a/src/libstore/worker-protocol.hh
+++ b/src/libstore/worker-protocol.hh
@@ -63,7 +63,7 @@ typedef enum {
 
 
 class Store;
-class Source;
+struct Source;
 
 Path readStorePath(Store & store, Source & from);
 template<class T> T readStorePaths(Store & store, Source & from);