cord tests: Suppress GCC warnings in 'de' app

cord tests: Suppress GCC warnings in 'de' app

* cord/tests/de.c (retrieve_line): Change type of "eol" local variable
from int to size_t (to prevent "comparison between signed and unsigned"
compiler warning).
* cord/tests/de_win.c (WinMain): Cast GetLastError() result to
unsigned int (to match printf format specifier).
* cord/tests/de_win.c (AboutBoxCallback): Suppress "unused parameter"
compiler warning for "lParam" (by casting the later to void).