travis: add gdb as dependency, it is used for cppcheck-gui.

former output:

{ test -n "" && DESTDIR="" || DESTDIR=.; } && test $(gdb --version | sed -e 's,[^0-9]\+\([0-9]\)\.\([0-9]\).*,\1\2,;q') -gt 72
&& gdb --nx --batch --quiet -ex 'set confirm off' -ex "save gdb-index $DESTDIR" -ex quit 'cppcheck-gui' && test -f
cppcheck-gui.gdb-index && objcopy --add-section '.gdb_index=cppcheck-gui.gdb-index' --set-section-flags '.gdb_index=readonly'
'cppcheck-gui' 'cppcheck-gui' && rm -f cppcheck-gui.gdb-index || true
/bin/sh: 1: gdb: not found
/bin/sh: 1: test: -gt: unexpected operator


fixes the "gdb not found" issue.