[Math] Fix variable length array warning in RMinimzer

Fix the following warning:

```txt
math/rtools/src/RMinimizer.cxx:28:19: error: variable length arrays in C++ are a Clang extension [-Werror,-Wvla-cxx-extension]
   28 |          double z[size];
      |                   ^~~~
math/rtools/src/RMinimizer.cxx:28:19: note: read of non-const variable 'size' is not allowed in a constant expression
math/rtools/src/RMinimizer.cxx:26:23: note: declared here
   26 |          unsigned int size = y.GetNoElements();
      |                       ^
```

Author: guitargeek

math/rtools/src/RMinimizer.cxx

@@ -25,9 +25,8 @@ namespace ROOT {
       TVectorD mingradfunction(TVectorD y){
          unsigned int size = y.GetNoElements();
          const double * yy = y.GetMatrixArray();
-         double z[size];
-         gGradFunction->Gradient(yy,z);
-         TVectorD zz(size,z);
+         TVectorD zz(size);
+         gGradFunction->Gradient(yy,zz.data());
          return zz;
       }