Fix build with clang on ppc64le (#2278)

* Fix build with clang on ppc64le

Clang doesn't implement vec_rsqrt. Code taken from https://github.com/opencv/opencv/blob/master/modules/core/include/opencv2/core/vsx_utils.hpp#L360

* Define vec_rsqrt only if we use VSX

dlib/simd/simd4f.h

@@ -524,6 +524,11 @@ namespace dlib
 
 // ----------------------------------------------------------------------------------------
 
+#if defined(DLIB_HAVE_VSX) && !defined(vec_rsqrt)
+    extern inline __vector float __attribute__((always_inline)) vec_rsqrt(const __vector float& a)
+    { return vec_div((__vector float){1, 1, 1, 1}, vec_sqrt(a)); }
+#endif
+
     inline simd4f reciprocal_sqrt (const simd4f& item)
     {
 #ifdef DLIB_HAVE_SSE2