Fix instances of '#if __LP64__'.

Triggers -Wundef, which is on in -Weverything.

Bug: http://b/31496165
Change-Id: Ib06107073f7dd1d584c19c222d0430da9d35630b
diff --git a/libc/private/bionic_ieee.h b/libc/private/bionic_ieee.h
index c579969..69095f0 100644
--- a/libc/private/bionic_ieee.h
+++ b/libc/private/bionic_ieee.h
@@ -79,7 +79,7 @@
   unsigned dbl_sign:1;
 };
 
-#if __LP64__
+#if defined(__LP64__)
 
 /* 64-bit Android uses ld128 long doubles. */