BUILD: debug: only dump/reset glitch counters when really defined

BUILD: debug: only dump/reset glitch counters when really defined

If neither DEBUG_GLITCHES nor DEBUG_STRICT is set, we end up with
no dbg_cnt section, resulting in debug_parse_cli_counters not
building due to __stop_dbg_cnt and __start_dbg_cnt not being defined.
Let's just condition the end of the function to these conditions.
An alternate approach (less elegant) is to always declare a dummy
entry of type DBG_COUNTER_TYPES in debug.c.

This must be backported to 3.1 since it was brought with glitches.