avoid adding "IsLittleEndian" utility code if it's not used

Cython/Compiler/FusedNode.py

@@ -560,8 +560,6 @@ class FusedCFuncDefNode(StatListNode):
         """
         from . import TreeFragment, Code, UtilityCode
 
-        env.use_utility_code(Code.UtilityCode.load_cached("IsLittleEndian","ModuleSetupCode.c"))
-
         fused_types = self._get_fused_base_types([
             arg.type for arg in self.node.args if arg.type.is_fused])
 
@@ -638,6 +636,7 @@ class FusedCFuncDefNode(StatListNode):
                     if normal_types:
                         self._fused_instance_checks(normal_types, pyx_code, env)
                     if buffer_types or pythran_types:
+                        env.use_utility_code(Code.UtilityCode.load_cached("IsLittleEndian", "ModuleSetupCode.c"))
                         self._buffer_checks(buffer_types, pythran_types, pyx_code, decl_code, env)
                     if has_object_fallback:
                         pyx_code.context.update(specialized_type_name='object')