[gssapi/gssproxy] cacb03: Add missing #include for musl

[David Seifert]

  Branch: refs/heads/main
  Home:   https://github.com/gssapi/gssproxy
  Commit: cacb03c5e0cb7fa5f4d20ce3e78fb067edc702ef
      https://github.com/gssapi/gssproxy/commit/cacb03c5e0cb7fa5f4d20ce3e78fb06...
  Author: David Seifert <soap(a)gentoo.org&gt;
  Date:   2022-01-24 (Mon, 24 Jan 2022)

  Changed paths:
    M src/gp_socket.c

  Log Message:
  -----------
  Add missing #include for musl

Bug: https://bugs.gentoo.org/719920

Signed-off-by: David Seifert <soap(a)gentoo.org&gt;