include/net: add the alignment for data in NET_BUF_POOL_FIXED_DEFINE

NET_BUF_POOL_FIXED_DEFINE locates net_buf_data in __noinit section,
it does not guarantees that data buffer will aligned.

There is wifi driver which required network buffers to be aligned.

Changes:
line below (from NET_BUF_POOL_FIXED_DEFINE macro):
 static uint8_t __noinit net_buf_data_##_name[_count][_data_size];
is updated to:
 static uint8_t __noinit net_buf_data_##_name[_count][_data_size]
 __net_buf_align;

Signed-off-by: Nazar Palamar <nazar.palamar@infineon.com>