tools/sd-notify.h: Include string.h too

Alpine Linux, when using --enable-systemd to get the init files, fails with:

  tools/include/xen-sd-notify.h:69:3: error: call to undeclared library
  function 'memcpy' with type 'void *(void *, const void *, unsigned long)';
  ISO C99 and later do not support implicit function declarations
  [-Wimplicit-function-declaration]
     69 |   memcpy(socket_addr.sun.sun_path, socket_path, path_length);
        |   ^

This will be down to using musl rather than glibc.  Include the appropriate
header.

Fixes: 78510f3a15 ("tools: Import stand-alone sd_notify() implementation from systemd")
Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>
(cherry picked from commit 8efff5a69a)

tools/include/xen-sd-notify.h

@@ -20,6 +20,7 @@
 #include <errno.h>
 #include <stddef.h>
 #include <stdlib.h>
+#include <string.h>
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <unistd.h>