libc: Do not redefine a typedef (C11 feature)

Summary:
Fixes the following:
bionic/libc/include/sys/un.h:32:23: warning: redefinition of typedef
'sa_family_t' is a C11 feature [-Wtypedef-redefinition]
typedef __sa_family_t sa_family_t;
                      ^~~~~~~~~~~
bionic/libc/include/sys/socket.h:46:23: note:
previous definition is here
typedef __sa_family_t sa_family_t;
                      ^~~~~~~~~~~

Test Plan: verfied

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D15