slabtop: separate header and slab list

[cakebaker]

This PR separates header and slab list with an empty line to match the output of procps-ng/slabtop.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 0.10%. Comparing base (9391cf1) to head (faca8fe).
Report is 6 commits behind head on main.

Files	Patch %	Lines
src/uu/slabtop/src/slabtop.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main     #59      +/-   ##
========================================
- Coverage   0.10%   0.10%   -0.01%     
========================================
  Files         18      18              
  Lines       1841    1842       +1     
  Branches     239     239              
========================================
  Hits           2       2              
- Misses      1839    1840       +1     

Flag	Coverage Δ
windows_latest	0.10% <0.00%> (-0.01%)	⬇️

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