Make add_separate_debug_objfile static

This changes objfile::make to take a "parent" parameter, and makes
add_separate_debug_objfile static.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tom@tromey.com>

	* symfile.c (symbol_file_add_with_addrs): Pass "parent" to
	objfile::make.
	* objfiles.h (struct objjfile) <make>: No longer inline.
	(add_separate_debug_objfile): Don't declare.
	* objfiles.c (add_separate_debug_objfile): Now static.
	(objfile::make): Move from objfiles.h.  Call
	add_separate_debug_objfile.  Add "parent" parameter.

Change-Id: I631f43bb71738dea6ae0697317bf8ef4a0db4617