Add dummy install target

The parent makefile will recursively call all submakefiles for various
targets among which install. So this makefile needs to handle that even
if it doesn't do anything

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5249

tests/Makefile

@@ -91,6 +91,9 @@ RUN_TESTS = $(addprefix run_, $(patsubst -,_,$(TESTS)))
 
 all: $(STAMPS) $(ARCHIVES) $(CONFIGS) $(VERSIONS) $(LATEST_VERSIONS)
 
+install:
+	# Empty target, nothing to install
+
 run_test-%: test-%
 	./$<