slirp: Fix build with gcc 9

Build fails with gcc 9:

  CC      slirp/ndp_table.o
slirp/ndp_table.c: In function ‘ndp_table_add’:
slirp/ndp_table.c:31:23: error: taking address of packed member of ‘struct ndpentry’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
   31 |         if (in6_equal(&ndp_table->table[i].ip_addr, &ip_addr)) {
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
slirp/ndp_table.c: In function ‘ndp_table_search’:
slirp/ndp_table.c:75:23: error: taking address of packed member of ‘struct ndpentry’ may result in an unaligned pointer value [-Werror=address-of-packed-member]
   75 |         if (in6_equal(&ndp_table->table[i].ip_addr, &ip_addr)) {
      |                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

The ndpentry structure isn't used to model on-the-wire data or anything
else that would care for the struct layout. It doesn't need to be packed
actually. Just drop SLIRP_PACKED.
Signed-off-by: NGreg Kurz <groug@kaod.org>
Message-Id: <155143315831.102868.17515265400523392682.stgit@bahia.lan>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NSamuel Thibault <samuel.thibault@ens-lyon.org>

slirp/slirp.h

@@ -106,7 +106,7 @@ bool arp_table_search(Slirp *slirp, uint32_t ip_addr,
 struct ndpentry {
     unsigned char   eth_addr[ETH_ALEN];     /* sender hardware address */
     struct in6_addr ip_addr;                /* sender IP address       */
-} SLIRP_PACKED;
+};
 
 #define NDP_TABLE_SIZE 16