package/pppd: disable for musl toolchains

package/pppd: disable for musl toolchains

For the same reason as for ipsec-tools [1], disable pppd for musl
toolchains.
It also include <sys/cdefs.h> which is not provided by musl [2].

This fixes a compilation error with musl libc because of undeclared
__P and missing sys/cdefs.h include.

Propagate the dependency to network-manager and rp-pppoe
packages.

[1] 33499484e457018203023cdfdd5286478f2ae381
[2] http://wiki.musl-libc.org/wiki/FAQ#Q:_I.27m_trying_to_compile_something_against_musl_and_I_get_error_messages_about_sys.2Fcdefs.h

Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Thomas: add Config.in comment in network-manager.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>