um: Avoid pcap multiple definition errors

Change the function name in pcap_kern to avoid conflicting with
libpcap.a.

e.g.
ld: /usr/lib/gcc/x86_64-pc-linux-gnu/12/../../../../lib64/libpcap.a(pcap.o): in function `pcap_init':
(.text+0x7f0): multiple definition of `pcap_init'; arch/um/drivers/pcap_kern.o:pcap_kern.c:(.text.unlikely+0x0): first defined here

Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Richard Weinberger <richard@nod.at>