Fix incorrect handling of ".include" in identifiers

[Mingtao521]

Souffle's lexer incorrectly recognizes ".include" as a keyword or directive, rather than part of an identifier.

Updated scanner.ll to ensure ".include" is only treated as a directive when followed by whitespace.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.06%. Comparing base (b82ae20) to head (9ce1be3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2498      +/-   ##
==========================================
- Coverage   81.11%   81.06%   -0.05%     
==========================================
  Files         492      492              
  Lines       30922    30922              
==========================================
- Hits        25082    25067      -15     
- Misses       5840     5855      +15     

Files	Coverage Δ
src/parser/scanner.ll	81.53% <100.00%> (ø)

... and 5 files with indirect coverage changes