zebra: fix compilation with GCC14

zebra: fix compilation with GCC14

Fixes:
zebra/zebra_netns_notify.c: In function 'zebra_ns_ready_read':
zebra/zebra_netns_notify.c:266:40: error: implicit declaration of function 'basename' [-Wimplicit-function-declaration]
  266 |         if (strmatch(VRF_DEFAULT_NAME, basename(netnspath))) {
      |                                        ^~~~~~~~

Fixed by including libgen.h, then since basename may modify its
parameter, allocate a copy on the stack, using strdupa, and pass the
temporary string to basename.

According to the man page for basename:
With glibc, one gets the POSIX version of basename() when
<libgen.h> is included, and the GNU version otherwise.

The POSIX version of basename may modify the contents of path,
so we should to pass a copy when calling this function.

[1] https://man7.org/linux/man-pages/man3/basename.3.html

Signed-off-by: Georgi Valkov <gvalkov@gmail.com>