build: fix for "make -j" safety

A make rule with multiple (non-pattern) targets just says that each of
those files can be built by the rule, not that the rule builds all of
them at once. So under "make -j", extensions/Makefile would run three
copies of glib-ginterface-gen.py at once, which could end up deleting
each others' files and causing a make failure. Fix.

Signed-off-by: Jonny Lamb <jonny.lamb@collabora.co.uk>

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