get rid of another compiler warning on Windows for %llu

get rid of another compiler warning on Windows for %llu

i686-w64-mingw32-gcc -c -pipe -O2 -DWINVER=0x501   -mwindows -Wall -Ipcre  parsers.c -o parsers.o
parsers.c: In function 'create_content_length_header':
parsers.c:4671:36: warning: unknown conversion type character 'l' in format [-Wformat=]
    snprintf(header, buffer_length, "Content-Length: %llu", content_length);
                                    ^
parsers.c:4671:36: warning: too many arguments for format [-Wformat-extra-args]