Fix -Wundef in _MSC_VER check.

Bug: http://b/31496165
Change-Id: Ia7acd464fd8dcc9cb88e61b4f4d369db49f5de76
Test: mma
diff --git a/include/jemalloc/jemalloc.h b/include/jemalloc/jemalloc.h
index 5ed13cb..c983dc8 100644
--- a/include/jemalloc/jemalloc.h
+++ b/include/jemalloc/jemalloc.h
@@ -127,7 +127,7 @@
 #  define JEMALLOC_CXX_THROW
 #endif
 
-#if _MSC_VER
+#if defined(_MSC_VER)
 #  define JEMALLOC_ATTR(s)
 #  define JEMALLOC_ALIGNED(s) __declspec(align(s))
 #  define JEMALLOC_ALLOC_SIZE(s)