Support user-provided DPI-C function declarations (#7626) (#7893)

Fixes #7626.

src/V3Task.cpp

@@ -1060,6 +1060,7 @@ class TaskVisitor final : public VNVisitor {
 
         // Add DPI Import to top, since it's a global function
         m_topScopep->scopep()->addBlocksp(funcp);
+        funcp->dpiCDecl(nodep->dpiCDecl());
         if (!makePortList(nodep, funcp)) return nullptr;
         return funcp;
     }