[fix compile warning] using format '%zu' for size_t

compile log:

khttp.c: In function 'khttp_send_http_req':
khttp.c:978:21: warning: format '%lu' expects argument of type 'long unsigned int', but argument 8 has type 'size_t' [-Wformat]

khttp.c: In function 'khttp_send_http_auth':
khttp.c:1242:21: warning: format '%lu' expects argument of type 'long unsigned int', but argument 16 has type 'size_t' [-Wformat]
[fix compile warning] remove useless local variable

compile log:
khttp.c:1483:19: warning: unused variable 'end' [-Wunused-variable]
            char *end = strstr(data, "\r\n\r\n");
                  ^
1 warning generated.
Merge pull request #2 from zlargon/fix_compile_warning

Fix compile warning