Consistently use `#if defined(__BIONIC__)`.

Test: treehugger
Change-Id: I844b909404532eb9165cba1ed6f237bdd4a46990
diff --git a/tests/poll_test.cpp b/tests/poll_test.cpp
index 33143f8..5799fea 100644
--- a/tests/poll_test.cpp
+++ b/tests/poll_test.cpp
@@ -49,7 +49,7 @@
 }
 
 TEST(poll, ppoll64_null_fds) {
-#if __BIONIC__
+#if defined(__BIONIC__)
   // Because nanosleep(2) is relatively new to POSIX, code sometimes abuses poll.
   errno = 0;
   timespec ts = { .tv_nsec = 100 };