md5: add "const" qualifier to the "file" argument

md5: add "const" qualifier to the "file" argument

This is intended to fix the following compiler warning in opkg-lede:

    /home/yousong/git-repo/lede-project/opkg-lede/libopkg/file_util.c: In function ‘file_md5sum_alloc’:
    /home/yousong/git-repo/lede-project/opkg-lede/libopkg/file_util.c:144:2: warning: passing argument 1 of ‘md5sum’ discards ‘const’ qualifier from pointer target type [enabled by default]
    In file included from /home/yousong/git-repo/lede-project/opkg-lede/libopkg/file_util.c:28:0:
    /home/yousong/.usr/include/libubox/md5.h:56:5: note: expected ‘char *’ but argument is of type ‘const char *’

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>