linux-yocto: add libmpc-native to DEPENDS

5.14 changed how the GCC plugins are built, which means they now
depend on both GMP and MPC to be built. We already depend on gmp-native,
so add libmpc-native aswell.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>