Reduce the exposure of the __set_errno implementation detail.

Change-Id: I395e1b46a9491e34fc53e71853e932ea90b3d1cc
diff --git a/libc/Android.mk b/libc/Android.mk
index 92bca4c..dc2b331 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -211,7 +211,6 @@
 	bionic/semaphore.c \
 	bionic/send.c \
 	bionic/setegid.c \
-	bionic/__set_errno.c \
 	bionic/seteuid.c \
 	bionic/setpgrp.c \
 	bionic/setresuid.c \
@@ -283,6 +282,7 @@
     bionic/__memcpy_chk.cpp \
     bionic/__memmove_chk.cpp \
     bionic/__memset_chk.cpp \
+    bionic/__set_errno.cpp \
     bionic/setlocale.cpp \
     bionic/__strcat_chk.cpp \
     bionic/__strcpy_chk.cpp \