To quash warnings from GCC 7.

	/home/yunion/git-repo/openwrt/openwrt/build_dir/target-mips_24kc_musl/linux-malta_be/opkg-2019-06-14-21b7bd73/libopkg/pkg_alternatives.c:35:42: warning: duplicate 'const' declaration specifier [-Wduplicate-decl-specifier]
	 static const struct alternative_provider const providers[] = {
						  ^~~~~
Signed-off-by: Yousong Zhou <zhouyousong@yunionyun.com>