gdb: Move declarations from symtab.h to source.h

gdb: Move declarations from symtab.h to source.h

Declarations for functions in source.c are split between source.h and
symtab.h.  This commit moves the small number that are in symtab.h
into source.h.  There's just one file that needs to add an include of
source.h in order to build.

I've moved the function header comments from source.c to source.h
inline with the recommended GDB style.

gdb/ChangeLog:

	* source.c (select_source_symtab): Move header comment to
	declaration in source.h.
	(forget_cached_source_info_for_objfile): Likewise.
	(forget_cached_source_info): Likewise.
	(identify_source_line): Likewise.
	* source.h (identify_source_line): Move declaration from symtab.h
	and add comment from source.c
	(print_source_lines): Likewise.
	(forget_cached_source_info_for_objfile): Likewise.
	(forget_cached_source_info): Likewise.
	(select_source_symtab): Likewise.
	(enum print_source_lines_flag): Move definition from symtab.h.
	* symtab.h (identify_source_line): Move declaration to source.h.
	(print_source_lines): Likewise.
	(forget_cached_source_info_for_objfile): Likewise.
	(forget_cached_source_info): Likewise.
	(select_source_symtab): Likewise.
	(enum print_source_lines_flag): Move definition to source.h.
	* tui/tui-hooks.c: Add 'source.h' include.