Roman Bogorodskiy reported a syntax-check failure when using
FreeBSD; complaining that:

prohibit_empty_first_line
tools/libvirt_win_icon_16x16.ico:1:
tools/libvirt_win_icon_32x32.ico:1:
tools/libvirt_win_icon_48x48.ico:1:
tools/libvirt_win_icon_64x64.ico:1:
maint.mk: Prohibited empty first line

In reality, the first 'line' of that file is NOT empty; but since
it is a binary file, awk is not required to handle it gracefully.
The simplest solution is to exempt all image files from syntax
checks in the first place - after all, we only store them in git
because they are inconvenient to regenerate, but they are not our
preferred format for making modifications, and syntax check should
only cover files that we are likely to modify.

* cfg.mk (VC_LIST_ALWAYS_EXCLUDE_REGEX): Exempt images.
(exclude_file_name_regexp--sc_prohibit_empty_lines_at_EOF): Simplify.
(exclude_file_name_regexp--sc_trailing_blank): Likewise.
Signed-off-by: NEric Blake <eblake@redhat.com>