Fix GCC 8 compiler warnings on strncpy's truncated copy on debian distro

Fix GCC 8 compiler warnings on strncpy's truncated copy on debian distro

For some reason, GCC 8 in debian is pickier than GCC 8 in ubuntu. It complains
about things in strncpy like this

/home/sluong/vpp/src/vlib/linux/pci.c:485:7: error: ‘strncpy’ output may be
truncated copying 15 bytes from a string of length 255 [-Werror=stringop-truncation]
       strncpy (ifr.ifr_name, e->d_name, sizeof (ifr.ifr_name) - 1);
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/sluong/vpp/src/vlib/linux/pci.c: At top level:

It also complains similar things in string_test.c

The fix in pci.c is to convert strncpy to use clib_strncpy
The fix in string_test.c is condiational compile the complained code for GCC 8.

Change-Id: Ic9341ca54ed7407210502197a28283bc42c26662
Signed-off-by: Steven Luong <sluong@cisco.com>