Fix duplicate include guard *ALTIVEC_H -> *ZVECTOR_H

Some some header guards were repeated between the `AltiVec` package and the
`ZVector` packages. This could cause a problem if (for whatever reason) someone
attempts to include headers for both architectures.
diff --git a/Eigen/src/Core/arch/ZVector/Complex.h b/Eigen/src/Core/arch/ZVector/Complex.h
index acb04fb..df5c8d4 100644
--- a/Eigen/src/Core/arch/ZVector/Complex.h
+++ b/Eigen/src/Core/arch/ZVector/Complex.h
@@ -8,8 +8,8 @@
 // Public License v. 2.0. If a copy of the MPL was not distributed
 // with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-#ifndef EIGEN_COMPLEX32_ALTIVEC_H
-#define EIGEN_COMPLEX32_ALTIVEC_H
+#ifndef EIGEN_COMPLEX32_ZVECTOR_H
+#define EIGEN_COMPLEX32_ZVECTOR_H
 
 #include "../../InternalHeaderCheck.h"
 
@@ -425,4 +425,4 @@
 
 } // end namespace Eigen
 
-#endif // EIGEN_COMPLEX32_ALTIVEC_H
+#endif // EIGEN_COMPLEX32_ZVECTOR_H
diff --git a/Eigen/src/Core/arch/ZVector/MathFunctions.h b/Eigen/src/Core/arch/ZVector/MathFunctions.h
index c86e185..bad16a6 100644
--- a/Eigen/src/Core/arch/ZVector/MathFunctions.h
+++ b/Eigen/src/Core/arch/ZVector/MathFunctions.h
@@ -13,8 +13,8 @@
  * Julien Pommier's sse math library: http://gruntthepeon.free.fr/ssemath/
  */
 
-#ifndef EIGEN_MATH_FUNCTIONS_ALTIVEC_H
-#define EIGEN_MATH_FUNCTIONS_ALTIVEC_H
+#ifndef EIGEN_MATH_FUNCTIONS_ZVECTOR_H
+#define EIGEN_MATH_FUNCTIONS_ZVECTOR_H
 
 #include "../../InternalHeaderCheck.h"
 
@@ -232,4 +232,4 @@
 
 }  // end namespace Eigen
 
-#endif  // EIGEN_MATH_FUNCTIONS_ALTIVEC_H
+#endif  // EIGEN_MATH_FUNCTIONS_ZVECTOR_H