Add space before messsage in byte compiler warnings

According to the GNU coding standards (info "(standards) Errors"):
> If you want to mention the column number, use one of these formats:
>      SOURCE-FILE-NAME:LINENO:COLUMN: MESSAGE
>      SOURCE-FILE-NAME:LINENO.COLUMN: MESSAGE

* lisp/emacs-lisp/bytecomp.el (byte-compile-warning-prefix): Add space
before message in byte compiler warnings to comply with the GNU coding
standards.  (Bug#18969)
1 job for master in 54 minutes and 43 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #4819
test-all

00:54:43