Fix void `string-join` function error

Upon running `'w3m-session-backup`, this error message would be printed:

    Symbol's function definition is void: string-join

I learned that Emacs doesn't load all functions at load. In particular
for this case, the `string-join` function was in a file not loaded by
default.

Thanks to the following resources:
https://emacs.stackexchange.com/questions/22142/cannot-open-load-file-subr-x
https://github.com/ralesi/ranger.el/blob/9db73d61/ranger.el#L84

I discovered I could require the `subr-x.el` file that contains the
`string-join` function needed. Decided to keep using `string-join`
instead of `(mapconcat 'identity ...` as it reads better in my mind.

1 files changed, 2 insertions, 0 deletions

diff --git a/w3m-session-backup.el b/w3m-session-backup.el
index 8301625..fc9e8af 100644
--- a/w3m-session-backup.el
+++ b/w3m-session-backup.el
@@ -32,6 +32,8 @@
 
 ;;; Code:
 
+(require 'subr-x)
+
 (defgroup w3m-session-backup nil
   "w3m-session-backup customisations.")