perf: make pkg-config available

Fixup the to be called pkg-config as $(CROSS_COMPILE)pkg-config doesn't
exist.
With kernels < 6.4 pkg-config is only used for the gtk+-2.0 integration
which is disabled in the OE config. Thus the issue wasn't exposed so
far. With kernel 6.4 pkg-config is additionally used for libtraceevent
and libtracefs.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>