Fix valgrind errors from CPP define already defined (r=edwsmith)

--HG--
extra : convert_revision : aaa4e162e47e7b05cf6c04a51975f6a4042c604c

js/src/nanojit/nanojit.h

@@ -90,7 +90,7 @@
 #endif
 #ifdef JS_VALGRIND
 #  include <valgrind/valgrind.h>
-#else
+#elif !defined(VALGRIND_DISCARD_TRANSLATIONS)
 #  define VALGRIND_DISCARD_TRANSLATIONS(addr, szB)
 #endif