- fix two build errors due to mismatch between function
  declarations and their definitions.
- silence two mismatched signs warnings via casts

```
vquic/ngtcp2.c:1690:10: error: conflicting types for 'Curl_quic_is_connected'
CURLcode Curl_quic_is_connected(struct connectdata *conn,
         ^
./quic.h:43:10: note: previous declaration is here
CURLcode Curl_quic_is_connected(struct connectdata *conn,
         ^
vquic/ngtcp2.c:1718:17: error: conflicting types for 'ng_process_ingress'
static CURLcode ng_process_ingress(struct connectdata *conn, int sockfd,
                ^
vquic/ngtcp2.c:90:17: note: previous declaration is here
static CURLcode ng_process_ingress(struct connectdata *conn,
                ^
vquic/ngtcp2.c:1733:37: warning: passing 'uint8_t [65536]' to parameter of type 'char *' converts between pointers to integer types with different
      sign [-Wpointer-sign]
    while((recvd = recvfrom(sockfd, buf, bufsize, 0,
                                    ^~~
/usr/local/opt/mingw-w64/toolchain-x86_64/x86_64-w64-mingw32/include/winsock2.h:1023:58: note: passing argument to parameter 'buf' here
  WINSOCK_API_LINKAGE int WSAAPI recvfrom(SOCKET s,char *buf,int len,int flags,struct sockaddr *from,int *fromlen);
                                                         ^
vquic/ngtcp2.c:1873:32: warning: passing 'uint8_t [1252]' to parameter of type 'const char *' converts between pointers to integer types with
      different sign [-Wpointer-sign]
    while((sent = send(sockfd, out, outlen, 0)) == -1 &&
                               ^~~
/usr/local/opt/mingw-w64/toolchain-x86_64/x86_64-w64-mingw32/include/winsock2.h:1027:60: note: passing argument to parameter 'buf' here
  WINSOCK_API_LINKAGE int WSAAPI send(SOCKET s,const char *buf,int len,int flags);
                                                           ^
2 warnings and 2 errors generated.
```