Make get_image_name static

The only callers of get_image_name are nat/windows-nat.c, so make it
static.

gdb/ChangeLog
2021-04-30  Tom Tromey  <tromey@adacore.com>

	* nat/windows-nat.h (get_image_name): Don't declare.
	* nat/windows-nat.c (get_image_name): Now static.