cppcheck: Make sure it's installed.

Change-Id: I1e33de118c8e022e63f4a2783927e05fe88c364c
Reviewed-on: https://code.wireshark.org/review/34305
Reviewed-by: Gerald Combs <gerald@wireshark.org>

tools/cppcheck/cppcheck.sh

@@ -25,6 +25,11 @@
 CPPCHECK=$(type -p cppcheck)
 CPPCHECK_DIR=$(dirname "$0")
 
+if [ -z "$CPPCHECK" ] ; then
+    echo "cppcheck not found"
+    exit 1
+fi
+
 THREADS=4
 LAST_COMMITS=0
 TARGET=""