esx: Fix gcc 4.6 warning about initialized but unused variables

This warnings come from partly generated code. Therefore, the best
solution is to mark them as potentially being unused using the
ATTRIBUTE_UNUSED macro. This is suggested by the gcc documentation.

Reported by Christophe Fergeau

src/esx/esx_vi.c

@@ -60,7 +60,7 @@
     void                                                                      \
     esxVI_##_type##_Free(esxVI_##_type **ptrptr)                              \
     {                                                                         \
-        esxVI_##_type *item = NULL;                                           \
+        esxVI_##_type *item ATTRIBUTE_UNUSED;                                 \
                                                                               \
         if (ptrptr == NULL || *ptrptr == NULL) {                              \
             return;                                                           \

src/esx/esx_vi_types.c

@@ -58,7 +58,7 @@
     void                                                                      \
     esxVI_##_type##_Free(esxVI_##_type **ptrptr)                              \
     {                                                                         \
-        esxVI_##_type *item = NULL;                                           \
+        esxVI_##_type *item ATTRIBUTE_UNUSED;                                 \
                                                                               \
         if (ptrptr == NULL || *ptrptr == NULL) {                              \
             return;                                                           \
@@ -189,7 +189,7 @@
     esxVI_##_type##_Cast##_dest_extra##FromAnyType(esxVI_AnyType *anyType,    \
                                                    _dest_type **ptrptr)       \
     {                                                                         \
-        _dest_type *item;                                                     \
+        _dest_type *item ATTRIBUTE_UNUSED;                                    \
                                                                               \
         if (anyType == NULL || ptrptr == NULL || *ptrptr != NULL) {           \
             ESX_VI_ERROR(VIR_ERR_INTERNAL_ERROR, "%s",                        \