preproc/html/pushback.cpp: variable 'i' [-Wunused-but-set-variable=]

[Bjarni Ingi Gislason]

../src/preproc/html/pushback.cpp: In member function 'int 
pushBackBuffer::readInt()':
../src/preproc/html/pushback.cpp:242:8: warning: variable 'c' set but not used 
[-Wunused-but-set-variable=]
  242 |   int  c =0;

And in line 262

    c++;

Warning is from g++-16 (Debian 16-20260308-1) 16.0.1
20260308 (experimental) [trunk r16-7949-g6c5de6335f5]