msdk: meson: fix pkg-config with sysroot
For building with some toolchains likes yocto, meson won't know the path return from pkg-config is a relative path to toolchain's sysroot. We would receive the following error:
sys/msdk/meson.build:72:2: ERROR: Include dir /usr/include/mfx does not exist.
Although we should blame the .pc files doesn't use ${pc_sysrootdir}. But cmake and autotools sound can handle it so I don't think Intel would accept such patch.