Makefile: fix use of -j without an argument

Makefile: fix use of -j without an argument

Our Makefile massages the given make arguments to invoke ninja
accordingly. One key difference is that ninja will parallelize by
default, whereas make only does so with -j<n> or -j. The make man page
says that "if the -j option is given without an argument, make will not
limit the number of jobs that can run simultaneously". We use to support
that by replacing -j with "" (empty string) when calling ninja, so that
it would do its auto-parallelization based on the number of CPU cores.

This was accidentally broken at d1ce2cc95b (Makefile: preserve
--jobserver-auth argument when calling ninja, 2024-04-02),
causing `make -j` to fail:

$ make -j V=1
  /usr/bin/ninja -v   -j -d keepdepfile all | cat
  make  -C contrib/plugins/ V="1" TARGET_DIR="contrib/plugins/" all
  ninja: fatal: invalid -j parameter
  make: *** [Makefile:161: run-ninja] Error

Let's fix that and indent the touched code for better readability.

Signed-off-by: Matheus Tavares Bernardino <quic_mathbern@quicinc.com>
Fixes: d1ce2cc95b ("Makefile: preserve --jobserver-auth argument when calling ninja", 2024-04-02)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>