configure: enable debug-mutex if debug enabled

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180425025459.5258-5-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -1042,6 +1042,7 @@ for opt do
   --enable-debug)
       # Enable debugging options that aren't excessively noisy
       debug_tcg="yes"
+      debug_mutex="yes"
       debug="yes"
       strip_opt="no"
       fortify_source="no"