ppp: update to 2.5.1

ppp: update to 2.5.1

ChangeLog:
https://github.com/ppp-project/ppp/blob/ppp-2.5.1/ChangeLog

Suppressed patches:
010-use_target_for_configure.patch [1]
510-pptp_compile_fix.patch [2]
520-u_int_bsd_fix.patch [3]

Upstreamed patches:
330-retain_foreign_default_routes.patch [4]
521-remove_unused_openssl_dep.patch [5]

[1] https://github.com/ppp-project/ppp/commit/e48a9b5de46b6d3a7fda98ac64de56092471ab98
[2] Merged into "500-add-pptp-plugin.patch"
[3] https://github.com/ppp-project/ppp/commit/797cdae57c27ed74a5ec3b4675f31f2c6cfc98ae
[4] https://github.com/ppp-project/ppp/commit/9856f47063c01096fe6649a21be9a021e42a45fe
[5] https://github.com/ppp-project/ppp/commit/59342ab622a96393d25d3a2f0592c8675e771295

Signed-off-by: Shiji Yang <yangshiji66@qq.com>
Link: https://github.com/openwrt/openwrt/pull/16605
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>