Place the Uint{32,64,128} and Int64 types in the internal namespace

These types are reserved for internal uses in simulators and/or
disassemblers. Let's not expose them to users.

Change-Id: I4e875d239b86e5a64b65f379f65c63efb4d6be9e
diff --git a/src/utils-vixl.h b/src/utils-vixl.h
index b0e3489..7bd1893 100644
--- a/src/utils-vixl.h
+++ b/src/utils-vixl.h
@@ -692,6 +692,8 @@
   std::vector<bool> bitfield_;
 };
 
+namespace internal {
+
 typedef int64_t Int64;
 class Uint64;
 class Uint128;
@@ -884,6 +886,8 @@
 
 Int64 BitCount(Uint32 value);
 
+}  // namespace internal
+
 }  // namespace vixl
 
 #endif  // VIXL_UTILS_H