ppp: compile fix: unset FILTER variable in Makefile

ppp: compile fix: unset FILTER variable in Makefile

If the environment variable FILTER is set before compilation,
compilation of the ppp-package will fail with the error message

Package ppp is missing dependencies for the following libraries:
libpcap.so.1

The reason is that the OpenWrt-patch for the Makefile only comments
out the line FILTER=y. Hence the pcap-library will be dynamically
linked if the environment variable FILTER is set elsewhere, which
causes compilation to fail. The fix consists on explicitly unsetting
the variable FILTER instead.

Signed-off-by: Eike Ritter <git@rittere.co.uk>
(cherry picked from commit 46cd0765d0c585dc0b48c8c0a3f116ef83cd580f)