Move __cmsg_nxthdr to its proper file.

Change-Id: I095fb6bc5318f2d256baa97658bdfe679d235aec
diff --git a/libc/Android.mk b/libc/Android.mk
index cfebd9c..b0a9fb1 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -107,7 +107,7 @@
     bionic/clearenv.cpp \
     bionic/clock.cpp \
     bionic/clone.cpp \
-    bionic/cmsg_nxthdr.cpp \
+    bionic/__cmsg_nxthdr.cpp \
     bionic/connect.cpp \
     bionic/__cxa_guard.cpp \
     bionic/__cxa_pure_virtual.cpp \