Declare libdconf_service as a dependency

This is the changes in bf103302

@ebassi let's review it in a merge request and merge it if it looks good.

From the commit message:

The dconf-service internal static library depends on generated files, and we need to make sure that those files are available by the time we build the tests that depend on them, as Meson builds everything in parallel. If we don't do that, in cases of massive parallelised builders, we will end up with errors like:

ccache cc -Itests/tests@@writer@exe -Itests -I../tests -I. -I../ -Itests/../service -I../tests/../service -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -fdiagnostics-color=always -pipe -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O0 -g '-DSYSCONFDIR="/etc"' -fno-common -Wmissing-prototypes -Wwrite-strings -O2 -g -m64 -mtune=generic '-DSRCDIR="/ostbuild/source/dconf/tests"'  -MD -MQ 'tests/tests@@writer@exe/writer.c.o' -MF 'tests/tests@@writer@exe/writer.c.o.d' -o 'tests/tests@@writer@exe/writer.c.o' -c ../tests/writer.c
../tests/writer.c:24:37: fatal error: service/dconf-generated.h: No such file or directory