dhcp: Don't use signed char for DHCP options

The value of DHCP_OPTEND is 255.  When it is assigned this result in a
sign change as the positive int constant is cast to a signed char and -1
results. Clang 4.0 complains about this.

src/libcharon/plugins/dhcp/dhcp_socket.c

@@ -176,7 +176,7 @@ typedef struct __attribute__((packed)) {
 	char server_hostname[64];
 	char boot_filename[128];
 	uint32_t magic_cookie;
-	char options[252];
+	u_char options[252];
 } dhcp_t;
 
 /**