utils/meas_pcap2db: fix -Wpointer-sign in pcap_cb()

When building with CFLAGS="-flto -Wall", I get the following:

  meas_pcap2db.c: In function ‘pcap_cb’:
  meas_pcap2db.c:64:27: warning: pointer targets in initialization of
                                 ‘const char *’ from ‘const u_char *’
                                 {aka ‘const unsigned char *’} differ
                                 in signedness [-Wpointer-sign]
     64 |         const char *cur = bytes;

Change-Id: I84d8e6f290bda0f03476f182f292ecc7a9e520f2

src/utils/meas_pcap2db.c

@@ -61,7 +61,7 @@ static void handle_mfm(const struct pcap_pkthdr *h,
 static void pcap_cb(u_char *user, const struct pcap_pkthdr *h,
 		   const u_char *bytes)
 {
-	const char *cur = bytes;
+	const u_char *cur = bytes;
 	const struct iphdr *ip;
 	const struct udphdr *udp;
 	const struct meas_feed_meas *mfm;