G-API: Fix compilation error in Standalone mode

- Point3f was added to type traits but was missing in the "own" package; fixed.

modules/gapi/include/opencv2/gapi/opencv_includes.hpp

@@ -31,6 +31,7 @@ namespace cv {
     using Size    = gapi::own::Size;
     using Point   = gapi::own::Point;
     using Point2f = gapi::own::Point2f;
+    using Point3f = gapi::own::Point3f;
     using Scalar  = gapi::own::Scalar;
     using Mat     = gapi::own::Mat;
 }  // namespace cv

modules/gapi/include/opencv2/gapi/own/types.hpp

@@ -43,6 +43,17 @@ public:
     float y = 0.f;
 };
 
+class Point3f
+{
+public:
+    Point3f() = default;
+    Point3f(float _x, float _y, float _z) : x(_x),  y(_y), z(_z) {}
+
+    float x = 0.f;
+    float y = 0.f;
+    float z = 0.f;
+};
+
 class Rect
 {
 public: