gdb: move read{now,never}_symbol_files declarations to symfile.h

... since they are defined in symfile.c.

gdb/ChangeLog:

	* top.h (readnow_symbol_files, readnever_symbol_files): Move
	declarations to ...
	* symfile.h: ... here.
	* symfile.c: Update doc.

Change-Id: Ie35a80d236bea70947bc496f66f62c8c621670b4