fix(autoware_mpc_lateral_controller): fix variableScope

[veqcc]

Description

Fixed the following cppcheck variableScope warning

control/autoware_mpc_lateral_controller/src/mpc.cpp:514:9: style: The scope of the variable 'idx_x_i_prev' can be reduced. [variableScope]
    int idx_x_i_prev = (i - 1) * DIM_X;
        ^

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How was this PR tested?

Notes for reviewers

None.

Interface changes

None.

Effects on system behavior

None.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.76%. Comparing base (5778e01) to head (9396b37).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9390      +/-   ##
==========================================
- Coverage   29.77%   29.76%   -0.01%     
==========================================
  Files        1338     1338              
  Lines      103364   103381      +17     
  Branches    40234    40237       +3     
==========================================
+ Hits        30773    30774       +1     
- Misses      69606    69621      +15     
- Partials     2985     2986       +1     

Flag	Coverage Δ		*Carryforward flag
differential	34.34% <100.00%> (?)
total	29.77% <ø> (-0.01%)	⬇️	Carriedforward from 5778e01

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