Fix instances of '#if __LP64__'.

Triggers -Wundef, which is on in -Weverything.

Bug: http://b/31496165
Change-Id: Ib06107073f7dd1d584c19c222d0430da9d35630b
diff --git a/libc/bionic/legacy_32_bit_support.cpp b/libc/bionic/legacy_32_bit_support.cpp
index f2bb37d..983fb32 100644
--- a/libc/bionic/legacy_32_bit_support.cpp
+++ b/libc/bionic/legacy_32_bit_support.cpp
@@ -37,7 +37,7 @@
 #include <sys/vfs.h>
 #include <unistd.h>
 
-#if __LP64__
+#if defined(__LP64__)
 #error This code is only needed on 32-bit systems!
 #endif