[libc++][Android] Include cstdlib to define __BIONIC__

__ANDROID__ is a built-in compiler macro, but __BIONIC__ is defined by
the libc header.

Reviewed By: #libc_abi, compnerd

Differential Revision: https://reviews.llvm.org/D137128

GitOrigin-RevId: f8c2b0d249afc58db87c6678e0db4c0a53774d2d

test/forced_unwind4.pass.cpp

diff --git a/test/forced_unwind4.pass.cpp b/test/forced_unwind4.pass.cpp
index 0d22657..2864426 100644
--- a/test/forced_unwind4.pass.cpp
+++ b/test/forced_unwind4.pass.cpp
@@ -13,6 +13,8 @@
 // the child_thread. This test ensures sigretrun is handled correctly (see:
 // UnwindCursor<A, R>::setInfoForSigReturn).
 
+#include <cstdlib> // defines __BIONIC__
+
 // Android/Bionic does not support pthread_cancel.
 #ifdef __BIONIC__
 int main() {