x11vnc: add libXtst as a dependency

x11vnc CAN be built without XTEST support, but it isn't very useful as
noted by the warning printed by configure when it isn't found:

*** A working build environment for the XTEST extension was not found ***
(libXtst).  An x11vnc built this way will be *ONLY BARELY USABLE*.
You will be able to move the mouse but not click or type.  There can
also be deadlocks if an application grabs the X server.

It is recommended that you install the necessary development packages
for XTEST (perhaps it is named something like libxtst-dev) and run
configure again.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>