openbsc/configure.ac: check for pcap/pcap.h.

The build process requires pcap/pcap.h in openbsc/src/utils/meas_pcap2db.c, but
the configure script did not check for that file.
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index 0ebb041..78302dd 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -88,6 +88,7 @@
 AC_HEADER_STDC
 AC_CHECK_HEADERS(dahdi/user.h,,AC_MSG_WARN(DAHDI input driver will not be built))
 AC_CHECK_HEADERS(dbi/dbd.h,,AC_MSG_ERROR(DBI library is not installed))
+AC_CHECK_HEADERS(pcap/pcap.h,,AC_MSG_ERROR(PCAP library is not installed))
 
 found_cdk=yes
 AC_CHECK_HEADERS(cdk/cdk.h,,found_cdk=no)