txring: fix __GLIBC_MINOR typo and include the header that defines the TX_RING API

[plusky]

The include guard in src/common/txring.h reads

#if __GLIBC__ >= 2 && __GLIBC_MINOR >= 1

but the macro is spelled __GLIBC_MINOR__. __GLIBC_MINOR is not defined by any header, so the preprocessor substitutes 0 and the condition is always false — the #else branch has been taken unconditionally on every glibc system since this check was added.

That accident is the only reason the file has ever compiled.

Why fixing the typo alone is not enough

<netpacket/packet.h>, the glibc header the "true" branch selects, defines only struct sockaddr_ll and struct packet_mreq. It has never provided the PACKET_TX_RING API. struct tpacket_hdr, struct tpacket_req and TPACKET_HDRLEN — all used by txring.h and txring.c — come only from <linux/if_packet.h>, which is in the #else branch. Correcting just the macro name gives:

error: storage size of 'h' isn't known        (struct tpacket_hdr)
error: 'TPACKET_HDRLEN' undeclared

So this PR fixes the include as well: use <linux/if_packet.h> in both branches and drop <netpacket/packet.h>, which supplies nothing this header needs.

A second bug this fixes

Before C23, <netpacket/packet.h> and <linux/if_packet.h> cannot be included together — both define struct sockaddr_ll and struct packet_mreq, and the __UAPI_DEF_* de-duplication does not apply:

linux/if_packet.h:14:8: error: redefinition of 'struct sockaddr_ll'
linux/if_packet.h:297:8: error: redefinition of 'struct packet_mreq'

configure's TX_RING probe includes both headers, so this collision makes the probe fail whenever tcpreplay is built with -std=gnu11 or -std=gnu17, silently disabling HAVE_TX_RING and falling back to the slower PF_PACKET send() path. Distributions that pin an older language level for the bundled (non-C23-clean) libopts hit exactly this — openSUSE has been shipping tcpreplay without TX_RING for this reason, and Fedora carries a partial patch for the typo that does not address the header problem.

After this change txring.h compiles under -std=gnu11, -std=gnu17 and -std=gnu23 alike.

Verification

Built on Linux/aarch64 with HAVE_TX_RING enabled (configure reports Linux TX_RING: yes), and run-tested: replayed frames were captured on a veth peer, 3/3 successful, 0 failed.