build: actually pass number of cores to ninja

Without this ninja supposedely builds with the default of the system
which is 10 for my 4 core, 8 thread CPU. With this argument, you can now
actually set it.

This has the benefit that the Gazebo plugins can be built with less
cores in order to avoid running out of RAM.

Makefile

@@ -92,9 +92,9 @@ ifdef NINJA_BUILD
 	PX4_MAKE := $(NINJA_BIN)
 
 	ifdef VERBOSE
-		PX4_MAKE_ARGS := -v
+		PX4_MAKE_ARGS := -j $(j) -v
 	else
-		PX4_MAKE_ARGS :=
+		PX4_MAKE_ARGS := -j $(j)
 	endif
 else
 	ifdef SYSTEMROOT