Deb: add gdb as a build dependency for stack traces on test suite failures

If mysqld crashed during the test suite run and gdb is available,
a stack trace will be printed automatically.

.travis.yml

@@ -21,6 +21,7 @@ addons:
       - debhelper
       - dh-apparmor
       - dpatch
+      - gdb
       - libaio-dev
       - libboost-dev
       - libjudy-dev

debian/control

@@ -9,6 +9,7 @@ Build-Depends: bison,
                dh-apparmor,
                dh-systemd,
                dpatch,
+               gdb,
                libaio-dev [linux-any],
                libboost-dev,
                libcrack2-dev (>= 2.9.0),