Fix instances of '#if __LP64__'.

Triggers -Wundef, which is on in -Weverything.

Bug: http://b/31496165
Change-Id: Ib06107073f7dd1d584c19c222d0430da9d35630b
diff --git a/libc/bionic/brk.cpp b/libc/bionic/brk.cpp
index a8c078b..e1a4b05 100644
--- a/libc/bionic/brk.cpp
+++ b/libc/bionic/brk.cpp
@@ -29,7 +29,7 @@
 #include <errno.h>
 #include <unistd.h>
 
-#if __LP64__
+#if defined(__LP64__)
 static void* __bionic_brk;
 #else
 void* __bionic_brk; // Accidentally exported by the NDK.