Fix a number of benign compiler warnings on Linux/GCC

[droe]

src/recv.c:103:17: warning: ‘static’ is not at beginning of declaration [-Wold-style-declaration]
  103 |                 const static uint32_t available_space = sizeof(fake_eth_hdr) - sizeof(struct ether_header);
      |                 ^~~~~

src/if-netmap-linux.c: In function ‘fetch_stats64’: src/if-netmap-linux.c:94:18: warning: ISO C99 doesn’t support unnamed structs/unions [-Wpedantic]
   94 |                 };
      |                  ^

src/probe_modules/packet.h: In function ‘check_dst_port’: src/probe_modules/packet.h:162:37: warning: suggest parentheses around comparison in operand of ‘^’ -Wparentheses]
  162 |                 return (to_validate <= max ^ to_validate >= min);
      |                         ~~~~~~~~~~~~^~~~~~

src/probe_modules/packet.c: In function ‘set_timestamp_option_with_nops’: src/probe_modules/packet.c:178:24: warning: implicit declaration of function ‘time’ [-Wimplicit-function-declaration]
  178 |         uint32_t now = time(NULL);
      |                        ^~~~
src/probe_modules/packet.c:178:24: warning: nested extern declaration of ‘time’ [-Wnested-externs]
  178 |         uint32_t now = time(NULL);
      |                        ^~~~