pcap: fix build with empty LIBPCAP_CFLAGS

pcap: fix build with empty LIBPCAP_CFLAGS

In rte.sdkbuild.mk with CONFIG_RTE_LIBRTE_PMD_PCAP=y,
we error-exit if LIBPCAP_CFLAGS is empty.
On some distros (e.g., Centos 6.4), it is normal for "pcap-config --cflags"
to output only a newline, because pcap header files reside in /usr/include/.
Solution is to remove the line that checks whether LIBPCAP_CFLAGS is empty.

Signed-off-by: Robert Sanford <rsanford@prolexic.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>