Hide __signalfd4, used to implement signalfd(3).

Bug: 11156955
Change-Id: I50842279cb5b32ec8bd45193435574e415cd806e
diff --git a/libc/bionic/signalfd.cpp b/libc/bionic/signalfd.cpp
index 36ef81d..63e1db4 100644
--- a/libc/bionic/signalfd.cpp
+++ b/libc/bionic/signalfd.cpp
@@ -30,9 +30,9 @@
 
 #include "private/kernel_sigset_t.h"
 
-extern "C" int signalfd4(int fd, kernel_sigset_t* mask, size_t sizemask, int flags);
+extern "C" int __signalfd4(int fd, kernel_sigset_t* mask, size_t sizemask, int flags);
 
 int signalfd(int fd, const sigset_t* mask, int flags) {
   kernel_sigset_t in_set(mask);
-  return signalfd4(fd, &in_set, sizeof(in_set), flags);
+  return __signalfd4(fd, &in_set, sizeof(in_set), flags);
 }