Add declaration for linux.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@177601 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/SDKs/linux/usr/include/stdlib.h b/SDKs/linux/usr/include/stdlib.h
index 2a6617a..e6b8bf7 100644
--- a/SDKs/linux/usr/include/stdlib.h
+++ b/SDKs/linux/usr/include/stdlib.h
@@ -22,6 +22,7 @@
 typedef __SIZE_TYPE__ size_t;
 
 void abort(void) __attribute__((__nothrow__)) __attribute__((__noreturn__));
+int atexit(void (*)(void)) __attribute__((__nothrow__));
 int atoi(const char *) __attribute__((__nothrow__));
 void free(void *) __attribute__((__nothrow__));
 char *getenv(const char *) __attribute__((__nothrow__))