tests: add swift_addNewDSOImage to the stubs

This is needed particularly on Windows which does not permit unresolved
symbols and the startup stub (swiftrt.obj) references the runtime
function.

Author: compnerd

validation-test/stdlib/MicroStdlib/Inputs/RuntimeStubs.c

@@ -18,3 +18,4 @@ void swift_retain(){}
 void swift_allocBox(){}
 void swift_getWitnessTable(void) {}
 void swift_getObjCClassMetadata(void) {}
+void swift_addNewDSOImage(void) {}