fping: Initialize msghdr struct in a portable way

Initializing the structure assuming glibc layout results in
compile errors on musl, therefore do partial intialization and then
assigning the members individually.

Signed-off-by: Khem Raj <raj.khem@gmail.com>

src/fping.c

@@ -1962,15 +1962,13 @@ int receive_packet(int64_t wait_time,
         reply_buf,
         reply_buf_len
     };
-    struct msghdr recv_msghdr = {
+    struct msghdr recv_msghdr = {0};
-        reply_src_addr,
+    recv_msghdr.msg_name = reply_src_addr;
-        reply_src_addr_len,
+    recv_msghdr.msg_namelen = reply_src_addr_len;
-        &msg_iov,
+    recv_msghdr.msg_iov = &msg_iov;
-        1,
+    recv_msghdr.msg_iovlen = 1;
-        &msg_control,
+    recv_msghdr.msg_control = &msg_control;
-        sizeof(msg_control),
+    recv_msghdr.msg_controllen = sizeof(msg_control);
-        0
-    };
 #if HAVE_SO_TIMESTAMPNS
     struct cmsghdr *cmsg;
 #endif