[Embedded] Add swift_getDefaultExecutor() to the list of functions to link.

We need to link `swift_getDefaultExecutor`, which is used from the C++
code.

rdar://156701386

Author: al45tair

lib/SILOptimizer/UtilityPasses/Link.cpp

@@ -105,6 +105,8 @@ class SILLinker : public SILModuleTransform {
                            SILLinkage::HiddenExternal);
     linkUsedFunctionByName("swift_createDefaultExecutors",
                            SILLinkage::HiddenExternal);
+    linkUsedFunctionByName("swift_getDefaultExecutor",
+                           SILLinkage::HiddenExternal);
     linkEmbeddedRuntimeWitnessTables();
   }