gdb/solib-svr4: make "lmid" XML attribute optional

When connecting to a GDBserver 12, which doesn't have support for
non-default linker namespaces and the "lmid" attribute in the
qxfer:libraries-svr4:read response, I get:

    (gdb) c
    Continuing.
    ⚠️  warning: while parsing target library list (at line 1): Required attribute "lmid" of <library> not specified

Given the code in library_list_start_library, I understand that the
"lmid" attribute is meant to be optional.  Mark it as optional in the
attribute descriptions, to avoid this warning.

Change-Id: Ieb10ee16e36bf8a771f944006e7ada1c10f6fbdc
Reviewed-By: Guinevere Larsen <guinevere@redhat.com>
diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
index f907571..e61aeaa 100644
--- a/gdb/solib-svr4.c
+++ b/gdb/solib-svr4.c
@@ -1115,7 +1115,7 @@ static const struct gdb_xml_attribute svr4_library_attributes[] =
   { "lm", GDB_XML_AF_NONE, gdb_xml_parse_attr_ulongest, NULL },
   { "l_addr", GDB_XML_AF_NONE, gdb_xml_parse_attr_ulongest, NULL },
   { "l_ld", GDB_XML_AF_NONE, gdb_xml_parse_attr_ulongest, NULL },
-  { "lmid", GDB_XML_AF_NONE, gdb_xml_parse_attr_ulongest, NULL },
+  { "lmid", GDB_XML_AF_OPTIONAL, gdb_xml_parse_attr_ulongest, NULL },
   { NULL, GDB_XML_AF_NONE, NULL, NULL }
 };