Fix a preprocessor warning exposed by -Wundef.

Bug: 18589470

__LP64__ is only defined for 64-bit compilation targets, so this macro
is evaluated as an implicit 0 in the original case. This change makes
it explicit that we only care whether this is actually defined.

Change-Id: I8c11b41fc0bde9477c5efeea0782d9741d9d827d
diff --git a/core/combo/include/arch/target_linux-x86/AndroidConfig.h b/core/combo/include/arch/target_linux-x86/AndroidConfig.h
index 6cdce4e..e294167 100644
--- a/core/combo/include/arch/target_linux-x86/AndroidConfig.h
+++ b/core/combo/include/arch/target_linux-x86/AndroidConfig.h
@@ -44,7 +44,7 @@
  * agree on the same size.  For desktop systems, use 64-bit values,
  * because some of our libraries (e.g. wxWidgets) expect to be built that way.
  */
-#if __LP64__
+#if defined(__LP64__)
 #define _FILE_OFFSET_BITS 64
 #endif