Fully initialize the custom_operations structs

Use also custom_compare_ext_default for the compare_ext field.

According to the git logs, this was introduced in OCaml 3.12.1, which is
earlier than out minimum required version.

mlaugeas is not touched by this change, since it is a copy of a 3rd
party library (and thus it will be fixed there first).

common/mlpcre/pcre-c.c

@@ -97,7 +97,8 @@ static struct custom_operations custom_operations = {
   custom_compare_default,
   custom_hash_default,
   custom_serialize_default,
-  custom_deserialize_default
+  custom_deserialize_default,
+  custom_compare_ext_default,
 };
 
 static value

common/mlprogress/progress-c.c

@@ -52,7 +52,8 @@ static struct custom_operations progress_bar_custom_operations = {
   custom_compare_default,
   custom_hash_default,
   custom_serialize_default,
-  custom_deserialize_default
+  custom_deserialize_default,
+  custom_compare_ext_default,
 };
 
 value

common/mlxml/xml-c.c

@@ -49,7 +49,8 @@ static struct custom_operations docptr_custom_operations = {
   custom_compare_default,
   custom_hash_default,
   custom_serialize_default,
-  custom_deserialize_default
+  custom_deserialize_default,
+  custom_compare_ext_default,
 };
 
 value
@@ -71,7 +72,8 @@ static struct custom_operations xpathctxptr_custom_operations = {
   custom_compare_default,
   custom_hash_default,
   custom_serialize_default,
-  custom_deserialize_default
+  custom_deserialize_default,
+  custom_compare_ext_default,
 };
 
 value
@@ -93,7 +95,8 @@ static struct custom_operations xpathobjptr_custom_operations = {
   custom_compare_default,
   custom_hash_default,
   custom_serialize_default,
-  custom_deserialize_default
+  custom_deserialize_default,
+  custom_compare_ext_default,
 };
 
 value

ocaml/guestfs-c.c

@@ -96,7 +96,8 @@ static struct custom_operations guestfs_custom_operations = {
   custom_compare_default,
   custom_hash_default,
   custom_serialize_default,
-  custom_deserialize_default
+  custom_deserialize_default,
+  custom_compare_ext_default,
 };
 
 static value

v2v/qemuopts-c.c

@@ -51,7 +51,8 @@ static struct custom_operations qemuopts_custom_operations = {
   custom_compare_default,
   custom_hash_default,
   custom_serialize_default,
-  custom_deserialize_default
+  custom_deserialize_default,
+  custom_compare_ext_default,
 };
 
 value