Fix LilyGo_TLora_V2_1_1_6_terminal_chat build

This change addresses two issues. The first is that the
LilyGo_TLora_V2_1_1_6_terminal_chat build would try to compile
simple_repeater/MyMesh.cpp. All other examples of terminal chat
targets are instead building simple_secure_chat/main.cpp . This
change would align this build to the rest of the builds.

The second issue, found during the course of investigating the
first, stems from simple_repeater/MyMesh.cpp using the
MAX_NEIGHBOURS #define to control whether the neighbor list is kept.
Repeaters that keep this list must define this value, and if the
value is not defined, then all neighbor-related functionality is
compiled out. However, the code that replies to
REQ_TYPE_GET_NEIGHBOURS did not properly check for this #define,
and thus any target that compiles simple_repeater/MyMesh.cpp
without defining MAX_NEIGHBOURS would get an undefined variable
compilation error.

As a practical matter though, there are no targets that compile
simple_repeater/MyMesh.cpp AND do not define MAX_NEIGHBOURS,
except this build due to the first issue. As a result, the
second issue is addressed only as a matter of completeness. The
expected behavior with this change is that such a repeater would
send a valid reply indicating zero known neighbors.

variants/lilygo_tlora_v2_1/platformio.ini

@@ -65,7 +65,7 @@ build_flags =
 ;  -D MESH_PACKET_LOGGING=1
 ;  -D MESH_DEBUG=1
 build_src_filter = ${LilyGo_TLora_V2_1_1_6.build_src_filter}
-  +<../examples/simple_repeater>
+  +<../examples/simple_secure_chat/main.cpp>
 lib_deps =
   ${LilyGo_TLora_V2_1_1_6.lib_deps}
   densaugeo/base64 @ ~1.4.0