gdb: move cmd_list_element::prefixname to cli/cli-decode.c

I don't think this method really benefits from being implemented in the
header file, especially because it's recursive, it can't be inlined.
Move it to the source file, so it's no re-compiled by every CU
including cli/cli-decode.h.

I also noticed this method could be const, make it so.

gdb/ChangeLog:

	* cli/cli-decode.h (prefixname): Make const, move implementation
	to cli/cli-decode.c.
	* cli/cli-decode.c (cmd_list_element::prefixname): New.

Change-Id: I1597cace98d9a4ba71f51f1f495e73cc07b5dcf3