pppd: Fix cross-compilation using Clang (#253)

Clang does not have the --print-sysroot option so the shell
snippet silently fails leading to "-I/usr/include/openssl".
Thankfully systems like Gentoo/portage or Yocto/bitbake enable
sysroot poisoning precisely to catch these kinds of bugs.

There is only one user of this non-standard CFLAG include in
pppcrypt.h, so make it consistent with the rest of the sources
(eg. see eap-tls.[h|c] openssl/* includes) and drop the fragile
sysroot hackery.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>