SSS_INI: remove 'const' specifier from getter

`sss_ini_get_string_config_value()` is a wrapper around
`ini_get_string_config_value()`, whose docs says
```
Returned value needs to be freed after use.
```
But an attempt to free 'const char *' results in discarded-qualifiers
warning.

src/tools/sssd_check_socket_activated_responders.c

@@ -30,7 +30,7 @@
 static errno_t check_socket_activated_responder(const char *responder)
 {
     errno_t ret;
-    const char *services;
+    char *services = NULL;
     const char *str;
     TALLOC_CTX *tmp_ctx;
     struct sss_ini *init_data;
@@ -90,6 +90,7 @@ static errno_t check_socket_activated_responder(const char *responder)
     ret = EOK;
 
 done:
+    free(services);
     talloc_free(tmp_ctx);
 
     return ret;

src/util/sss_ini.c

@@ -395,7 +395,7 @@ int sss_ini_get_int_config_value(struct sss_ini *self,
 
 /* Get string value */
 
-const char *sss_ini_get_string_config_value(struct sss_ini *self,
+char *sss_ini_get_string_config_value(struct sss_ini *self,
                                             int *error)
 {
     return ini_get_string_config_value(self->obj, error);

src/util/sss_ini.h

@@ -136,7 +136,7 @@ int sss_ini_get_int_config_value(struct sss_ini *self,
 /**
  * @brief Get string value
  */
-const char *sss_ini_get_string_config_value(struct sss_ini *self,
+char *sss_ini_get_string_config_value(struct sss_ini *self,
                                             int *error);
 
 /**