meson: set the sysroot in the cross files

When cross compiling we have a sysroot, so by telling Meson where it is
explicitly it can do the right thing.

Currently this means it just sets PKG_CONFIG_SYSROOT_DIR to the value
that we already set it to, but in the future this can be used to stop
Meson looking directly in /usr when it shouldn't.

(From OE-Core rev: 62d4fd215a0886056d64e926f722cd07e9986118)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes-recipe/meson.bbclass

@@ -90,6 +90,7 @@ cpp_link_args = ${@meson_array('LDFLAGS', d)}
 
 [properties]
 needs_exe_wrapper = true
+sys_root = '${STAGING_DIR_HOST}'
 
 [host_machine]
 system = '${@meson_operating_system('HOST_OS', d)}'