libpurple/Makefile.am: Fix dbus code generation from VPATH

When building libpurple in a VPATH (an external directory such as when
running distcheck), make sometimes finds enums.h and marshallers.h in
the source directory, but dbus-analyze-types.py doesn't, because it's
only looking in the build directory.

This patch resolves this problem by having dbus-analyze-types.py not
scan enums.h nor marshallers.h. They don't actually generate anything
anyway.

However, it also adds enums.h and marshallers.h as dependencies to
the $(libpurple_la_OBJECTS) rule so that `make distcheck` directly
after configuring still works.