Move t_step_string declaration outside of MPI block

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: danieljvickers

src/post_process/m_data_input.f90

@@ -109,13 +109,13 @@ impure subroutine s_read_ib_data_files(file_loc_base, t_step)
         character(LEN=path_len + 2*name_len) :: file_loc
         logical                              :: file_exist
         integer                              :: ifile, ierr, data_size
+        character(len=10)                    :: t_step_string
 
 #ifdef MFC_MPI
         integer, dimension(MPI_STATUS_SIZE) :: status
         integer(KIND=MPI_OFFSET_KIND)       :: disp
         integer(KIND=MPI_OFFSET_KIND)       :: m_MOK, n_MOK, p_MOK, MOK, WP_MOK, var_MOK
         integer                             :: save_index
-        character(len=10)                   :: t_step_string
 #endif
 
         if (.not. ib) return