Parallel build fix: interposer needs libs.

Without this, a parallel build will sometimes fail in source/interposer with:
g++: error: ../../build/modules/Release/juce_core.a: No such file or directory

Makefile

@@ -150,7 +150,7 @@ bridges-ui: libs
 discovery: libs
 	@$(MAKE) -C source/discovery
 
-interposer:
+interposer: libs
 	@$(MAKE) -C source/interposer
 
 plugin: backend bridges-plugin bridges-ui discovery