package/libpcap: fix build failure without wchar

package/libpcap: fix build failure without wchar

libpcap requires c99 since version 1.10.0 and
https://github.com/the-tcpdump-group/libpcap/commit/68e5ca3f4cbb66d687582ae9563009c87bf3d184

However, this test will fail without wchar on:

configure:3609: /home/buildroot/autobuild/run/instance-2/output-1/host/bin/arm-linux-gcc -std=gnu99 -c -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O2   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 conftest.c >&5
conftest.c:52:9: error: unknown type name 'wchar_t'
   const wchar_t *name;
         ^~~~~~~

So hardcode ac_cv_prog_cc_c99 as already done in numerous package such
as apparmor, wireshark, xz, etc.

Fixes:
 - http://autobuild.buildroot.org/results/6e87c25a0dccccf5e7ad03742597a0613f554506

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>