Move Vlibrary_cache to emacs.c and reset before dumping.

Move Vlibrary_cache to emacs.c and reset before dumping.

* src/lisp.h (reset_image_types): Declare.
[WINDOWSNT] (Vlibrary_cache): Declare.

* src/image.c (reset_image_types): New function.

* src/emacs.c [WINDOWSNT] (Vlibrary_cache): Move from w32.c.
(syms_of_emacs) [WINDOWSNT] <Vlibrary_cache>: Initialize and staticpro.
(Fdump_emacs): Reset Vlibrary_cache and image_types.

* src/w32.c (Vlibrary_cache): Do not define; moved to emacs.c
(globals_of_w32) <Vlibrary_cache>: Do not initialize.

* src/w32.h (Vlibrary_cache): Do not declare.