ppc: util_altivec: Surround AltiVec-related code by appropriate ifdefs

This prevents non-AltiVec-enabled compilers from choking.

libavutil/ppc/util_altivec.h

@@ -34,6 +34,8 @@
 
 #include "types_altivec.h"
 
+#if HAVE_ALTIVEC
+
 // used to build registers permutation vectors (vcprm)
 // the 's' are for words in the _s_econd vector
 #define WORD_0 0x00,0x01,0x02,0x03
@@ -115,4 +117,6 @@ static inline vec_u8 load_with_perm_vec(int offset, uint8_t *src, vec_u8 perm_ve
     return vec_perm(a, b, perm_vec);
 }
 
+#endif /* HAVE_ALTIVEC */
+
 #endif /* AVUTIL_PPC_UTIL_ALTIVEC_H */