Simplify calls to init_psymbol_list

Existing callers to init_psymbol_list were checking to see if psymbols
had already been initialized.  It seemed better to me to do this check
directly in init_psymbol_list, simplifying the callers.

gdb/ChangeLog
2019-01-10  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (xcoff_initial_scan): Unconditionally call
	init_psymbol_list.
	* psymtab.c (init_psymbol_list): Do nothing if already called.
	* psympriv.h (init_psymbol_list): Add comment.
	* dwarf2read.c (dwarf2_build_psymtabs): Unconditionally call
	init_psymbol_list.
	* dbxread.c (dbx_symfile_read): Unconditionally call
	init_psymbol_list.