Move some declarations to source.h

Move some declarations to source.h

I noticed a few declarations in defs.h that really could be put into
source.h.  I think it's generally preferable to something out of
defs.h unless it is needed by most of the files in gdb.

gdb/ChangeLog
2018-02-14  Tom Tromey  <tom@tromey.com>

	* solib.c: Include source.h.
	* nto-tdep.c: Include source.h.
	* mi/mi-cmd-env.c: Include source.h.
	* infcmd.c: Include source.h.
	* exec.c: Include source.h.
	* defs.h (enum openp_flag, openp, source_full_path_of, mod_path)
	(add_path, directory_switch, source_path, init_source_path): Move
	declarations...
	* source.h (enum openp_flag, openp, source_full_path_of, mod_path)
	(add_path, directory_switch, source_path, init_source_path):
	...here.