meson.build: replace get_pkgconfig_variable with get_variable

meson.build: replace get_pkgconfig_variable with get_variable

Newer meson requires using get_variable(pkgconfig: ...) instead of
get_pkgconfig_variable() and results in this error:

| ../git/homescreen/meson.build:23: WARNING: Project targets '>= 0.60.0' but uses feature deprecated since '0.56.0': dependency.get_pkgconfig_variable. use dependency.get_variable(pkgconfig : ...) instead

Also, look for wayland-scanner program directly, instead of querying
pkgconfig, so it works properly with OE recipe-specific sysroot.

Bug-AGL: SPEC-4578

Signed-off-by: Denys Dmytriyenko <denys@konsulko.com>
Change-Id: I57abc17abea31883c9cc63419a0c2a868e262bba