Fix missing "re" module import in GDB support script

The gdb-v8-support.py script uses the "re" module to match regular expression,
but it does not import it, resulting in an error when loading the script from
GDB. This patch adds the missing import.

BUG=

Review URL: https://codereview.chromium.org/1245713002

Cr-Commit-Position: refs/heads/master@{#29770}

tools/gdb-v8-support.py

@@ -25,6 +25,7 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+import re
 
 kSmiTag = 0
 kSmiTagSize = 1