[libomptarget] Fix compilation with libc++

iterator is only guaranteed to be default-constructible, without any argument.

Differential Revision: https://reviews.llvm.org/D29171

llvm-svn: 293277

openmp/libomptarget/src/omptarget.cpp

@@ -72,7 +72,7 @@ struct LookupResult {
 
   HostDataToTargetListTy::iterator Entry;
 
-  LookupResult() : Flags({0,0,0}), Entry(0) {}
+  LookupResult() : Flags({0,0,0}), Entry() {}
 };
 
 /// Map for shadow pointers