Fix missing include

Author: Meinersbur

flang/lib/Lower/Allocatable.cpp

@@ -37,6 +37,7 @@
 #include "flang/Runtime/pointer.h"
 #include "flang/Semantics/tools.h"
 #include "flang/Semantics/type.h"
+#include "mlir/Dialect/OpenMP/OpenMPDialect.h"
 #include "mlir/Dialect/OpenMP/OpenMPInterfaces.h"
 #include "llvm/Support/CommandLine.h"