build: Create the storage directory at install time

Otherwise you could get into a state where the daemon could not start
because the directory listed as a ReadWritePaths in the .service file is
missing.

Spotted by Will Thompson.

See: !5

src/Makefile.am

@@ -50,3 +50,7 @@ fprintd-marshal.h: fprintd-marshal.list
 fprintd-marshal.c: fprintd-marshal.h
 	( $(GLIB_GENMARSHAL) --prefix=fprintd_marshal $(srcdir)/fprintd-marshal.list --body --header > fprintd-marshal.c )
 
+install-data-hook:
+	if test -w $(DESTDIR)$(prefix)/; then \
+		mkdir -p $(DESTDIR)$(localstatedir)/lib/fprint; \
+	fi