apk: add missing PROVIDES

Add PROVIDES:=apk to the Makefile so we can default to install `apk` by
default as a dependency.

Signed-off-by: Paul Spooren <mail@aparcar.org>

package/system/apk/Makefile

@@ -30,6 +30,7 @@ define Package/apk/default
   TITLE:=apk package manager
   DEPENDS:=+zlib
   URL:=$(PKG_SOURCE_URL)
+  PROVIDES:=apk
 endef
 
 define Package/apk-mbedtls