fix 'make' header dependency generation

'make depend' was being run before build-time generated headers
were created (with generate_uudmap). This meant that the 'cc -E' which the
dependency analysis ran would fail for some source files, since it
couldn't find some of the headers. It would thus fail to find any further
dependencies for that file after that point.

In particular, globals.o wasn't being marked as dependent on opcode.h,
even though opcode.h contains the definitions of some global vars.

The fix is simple. Make the 'depend' target depend on $(generated_headers).