Make 'patch' a proper prerequisite of the libdgl build

The patch step modifies lib/DPF/dgl/Widget.hpp (adds 'virtual' to
setVisible), and the dpf target compiles libdgl from those headers.
Previously 'all: patch build generate_ttl' listed them as siblings,
which serial make handles correctly (prerequisites processed
left-to-right) but parallel make does not: with -j N, patch and dpf
could run concurrently and libdgl could be built against unpatched
headers.

Add 'dpf: patch' to express the actual dependency, and drop 'patch'
from all's prereq list since it's now reached transitively via
all -> build -> common -> dpf -> patch.

This lets downstream packagers run plain 'make -j N' without an
explicit 'make patch' first. Verified by full parallel rebuild
from a clean tree: Widget.hpp ends up patched, libdgl builds
against patched headers, all four plugin formats build, TTL files
generate.

Author: magnetophon

plugin/dpf/Makefile

@@ -1,4 +1,4 @@
-all: patch build generate_ttl
+all: build generate_ttl
 
 build: common DigiDrie
 
@@ -67,7 +67,7 @@ datadir ?= $(datarootdir)
 docdir ?= $(datarootdir)/doc
 
 .PHONY: dpf
-dpf:
+dpf: patch
 	$(MAKE) -C lib/DPF HAVE_CAIRO=$(HAVE_CAIRO) USE_WEB_VIEW=false dgl utils/lv2_ttl_generator
 
 .PHONY: common