Add missing symbol imports to meijerint benchmark
Add missing symbol imports to meijerint benchmark

Symbols a, b, s, z were used but not defined, resulting in errors.

Also, "cosint" function was invoked, but there is no SymPy function by
that name (the cosine integral is Ci, which was already benchmarked). Based
on context, I guess that Chi was intended.

kaushik94/sympy,kaushik94/sympy,kaushik94/sympy