busybox: Move libresolv detection to LEDE Makefile

Since the LEDE nslookup applet is already specific to LEDE, move the
libresolv detection into the busybox Makefile that LEDE uses.

This fixes builds with external toolchains that don't automatically
search for headers and/or libraries without being told so.

Fixes: de5b8e5d ("busybox: add musl compatible nslookup replacement")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>