initialize in a separate line to avoid LFortran warning

[gxyd]

Description

This removes a minor warning when running tests with LFortran, the warning is:

warning: Assuming implicit save attribute for variable declaration
   --> ../src/mpi.f90:144:27
    |
144 |         integer(c_int) :: argc = 0
    |                           ^^^^^^^^ help: add explicit save attribute or parameter attribute or initialize in a separate statement


Note: Please report unclear, confusing or incorrect messages as bugs at
https://github.com/lfortran/lfortran/issues.

which doesn't occur anymore.