civetweb.c: %n in format expect 'int *' and not 'uint *'

Change len from unsigned int to int to fix usage of %n.

[src/civetweb.c:5616]: (warning) %n in format string (no. 6) requires
 'int *' but the argument type is 'unsigned int *'.
[src/civetweb.c:5628]: (warning) %n in format string (no. 2) requires
 'int *' but the argument type is 'unsigned int *'.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>