Fix build error with non C99 builds

module-webif.c: In function ‘send_oscam_scanusb’: module-webif.c:7167:2: error: ‘for’ loop initial declarations are only allowed in C99 mode for (uint8_t i = 0; i < (sizeof(elems) / sizeof(elems[0])); i+=5) ^ module-webif.c:7167:2: note: use option -std=c99 or -std=gnu99 to compile your code