Add missing override keyword

[luc-tielen]

This fixes an error I'm seeing when building souffle-haskell for OSX.

       > /tmp/nix-build-souffle-2.3.drv-0/source/src/main.cpp:306:10: error: 'endInput' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]
       >     bool endInput() {
       >          ^
       > /tmp/nix-build-souffle-2.3.drv-0/source/src/main.cpp:213:18: note: overridden virtual function is here
       >     virtual bool endInput() = 0;
       >                  ^
       > 1 error generated.
       > ninja: build stopped: subcommand failed.

[codecov]

Codecov Report

Merging #2279 (f1962b4) into master (bec3083) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2279      +/-   ##
==========================================
- Coverage   76.92%   76.91%   -0.01%     
==========================================
  Files         455      455              
  Lines       28658    28658              
==========================================
- Hits        22045    22043       -2     
- Misses       6613     6615       +2     

Impacted Files	Coverage Δ
src/main.cpp	68.62% <100.00%> (ø)
src/include/souffle/utility/ParallelUtil.h	83.72% <0.00%> (-1.56%)	⬇️

[b-scholz]

Thanks for the fix!