Fix test

Author: NimishMishra

flang/test/Lower/OpenMP/atomic-implicit-cast.f90

@@ -1,6 +1,6 @@
 ! REQUIRES : openmp_runtime
 
-! RUN: %flang_fc1 -emit-hlfir %openmp_flags %s -o - | FileCheck %s
+! RUN: %flang_fc1 -emit-hlfir -fopenmp %openmp_flags %s -o - | FileCheck %s
 
 ! CHECK: func.func @_QPatomic_implicit_cast_read() {
 subroutine atomic_implicit_cast_read