#1845 Scintilla: gtk/makefile minor improvement.

[Van de Bugger]

Currently, if gtk/deps.mak is missed for any reason, make fails:

$ make
makefile:120: deps.mak: No such file or directory
make: *** No rule to make target 'deps.mak'.  Stop.

Attempt to recreate deps.mak also fails:

$ make deps
makefile:120: deps.mak: No such file or directory
make: *** No rule to make target 'deps.mak'.  Stop.

Proposed minor fix let GNU make automatically recreate missed deps.mak file.

[Neil Hodgson]

Committed as [48e71f].