Fix unused results warnings
Fixes:
uclient-http.c:385:8: error: ignoring return value of 'fread', declared with attribute warn_unused_result [-Werror=unused-result]
fread(&val, sizeof(val), 1, f);
^
uclient-fetch.c: In function 'main':
uclient-fetch.c:664:12: error: ignoring return value of 'asprintf', declared with attribute warn_unused_result [-Werror=unused-result]
asprintf(&auth_str, "%s:%s", username, password);
^
uclient-fetch.c: In function 'read_data_cb':
uclient-fetch.c:269:9: error: ignoring return value of 'write', declared with attribute warn_unused_result [-Werror=unused-result]
write(output_fd, buf, len);
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>