Fix structName for single letter structs

[bernhardmgruber]

Fixes:

struct A{};
llama::structName<A>(); // compile-error

[codecov]

Codecov Report

Merging #800 (3800690) into develop (f4c17b9) will increase coverage by 0.00%.
Report is 5 commits behind head on develop.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #800   +/-   ##
========================================
  Coverage    98.79%   98.79%           
========================================
  Files           75       75           
  Lines         7318     7320    +2     
========================================
+ Hits          7230     7232    +2     
  Misses          88       88