Fix a compilation error with clang-cl (VS2022)

[mrexodia]

D:\CodeBlocks\cxx-common-cmake\build\z3-prefix\src\z3\src\util/vector.h(741,35): error: reference to 'vector' is ambiguous
    svector(SZ s, T const & elem):vector<T, false, SZ>(s, elem) {}
                                  ^
D:\CodeBlocks\cxx-common-cmake\build\z3-prefix\src\z3\src\util/vector.h(163,7): note: candidate found by name lookup is 'vector'
class vector {
      ^
c:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.34.31933\include\vector(494,7): note: candidate found by name lookup is 'std::vector'
class vector { // varying size array of values
      ^
In file included from D:\CodeBlocks\cxx-common-cmake\build\z3-prefix\src\z3\src\math\polynomial\upolynomial_factorization.cpp:27:
In file included from D:\CodeBlocks\cxx-common-cmake\build\z3-prefix\src\z3\src\math/polynomial/upolynomial_factorization_int.h:28:
In file included from D:\CodeBlocks\cxx-common-cmake\build\z3-prefix\src\z3\src\math/polynomial/upolynomial_factorization.h:27:
In file included from D:\CodeBlocks\cxx-common-cmake\build\z3-prefix\src\z3\src\math/polynomial/upolynomial.h:26:
In file included from D:\CodeBlocks\cxx-common-cmake\build\z3-prefix\src\z3\src\util/mpzzp.h:28:
In file included from D:\CodeBlocks\cxx-common-cmake\build\z3-prefix\src\z3\src\util/mpz.h:27:

I'm not exactly sure why this is erroring (the using is after the headers after all), but using namespace std; is bad practice regardless and this seems to be almost the only place that has it.