Increment SHLVL before launching tmux

The incrementing of SHLVL is configuration dependent. It is best to
always increment it to prevent infinite loops.

This reverts commit 17a4505a0a.

plugins/tmux/init.zsh

@@ -17,6 +17,8 @@ alias tl="tmux list-sessions"
 
 # Auto Start
 if (( $SHLVL == 1 )) && zstyle -t ':omz:plugin:tmux:auto' start; then
+  (( SHLVL += 1 )) && export SHLVL
+
   session="$(
     tmux list-sessions 2> /dev/null \
       | cut -d':' -f1 \