fdtrack: add wrapper for socketpair.

Bug: https://issuetracker.google.com/154450436
Test: bionic-unit-tests
Change-Id: I8b25accf00dc01f6fab351f3ba612f6b0ff9d094
(cherry picked from commit b107eab5ef1d5946bd753cad0e8b178172d362df)
diff --git a/libc/Android.bp b/libc/Android.bp
index 0446abc..9086de2 100644
--- a/libc/Android.bp
+++ b/libc/Android.bp
@@ -1117,6 +1117,7 @@
         "bionic/signal.cpp",
         "bionic/sigprocmask.cpp",
         "bionic/sleep.cpp",
+        "bionic/socketpair.cpp",
         "bionic/spawn.cpp",
         "bionic/stat.cpp",
         "bionic/stdlib_l.cpp",