fix build on linux

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@78538 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/endianness.h b/lib/endianness.h
index 8069bac..5c212c4 100644
--- a/lib/endianness.h
+++ b/lib/endianness.h
@@ -72,7 +72,7 @@
 
 /* .. */
 
-#if defined(__Linux__)
+#if defined(__linux__)
 #include <endian.h>
 
 #if __BYTE_ORDER == __BIG_ENDIAN