relibc: add Linux struct ifreq and ifconf to net/if.h

Qt 6.11.1 qtbase/qnetworkinterface_unix.cpp needs struct ifreq and
struct ifconf for SIOCGIFMTU/SIOCGIFINDEX/SIOCGIFCONF ioctls. Define
them in the cbindgen-generated net/if.h with sys/socket.h included.

src/header/net_if/cbindgen.toml

@@ -8,5 +8,48 @@ style = "Tag"
 no_includes = true
 cpp_compat = true
 
+after_includes = """
+#include <sys/socket.h>
+
+/* Nonstandard Linux netdevice structures used by ioctl (SIOCGIFMTU, etc.).
+   See https://man7.org/linux/man-pages/man7/netdevice.7.html. */
+
+struct ifmap {
+    unsigned long mem_start;
+    unsigned long mem_end;
+    unsigned short base_addr;
+    unsigned char irq;
+    unsigned char dma;
+    unsigned char port;
+};
+
+struct ifreq {
+    char ifr_name[16];
+    union {
+        struct sockaddr ifr_addr;
+        struct sockaddr ifr_dstaddr;
+        struct sockaddr ifr_broadaddr;
+        struct sockaddr ifr_netmask;
+        struct sockaddr ifr_hwaddr;
+        short ifr_flags;
+        int ifr_ifindex;
+        int ifr_metric;
+        int ifr_mtu;
+        struct ifmap ifr_map;
+        char ifr_slave[16];
+        char ifr_newname[16];
+        char *ifr_data;
+    };
+};
+
+struct ifconf {
+    int ifc_len;
+    union {
+        char *ifc_buf;
+        struct ifreq *ifreq;
+    };
+};
+"""
+
 [enum]
 prefix_with_name = true