compiler.h: Avoid duplicate definition of __unused

Avoid duplicate definition of __unused macro in environments
that already define the macro.

Signed-off-by: Moritz Fischer <moritzf@google.com>
Change-Id: Ic91623d67325cb5eb25fd111416c4ef5968a9e96
diff --git a/compiler.h b/compiler.h
index 6aefbf9..18bccee 100644
--- a/compiler.h
+++ b/compiler.h
@@ -2,6 +2,8 @@
 #ifndef _COMPILER_H
 #define _COMPILER_H
 
+#ifndef __unused
 #define __unused __attribute__((unused))
+#endif
 
 #endif