ola: fix host build by inheriting host PKG_CONFIG_PATH

ola: fix host build by inheriting host PKG_CONFIG_PATH

The ola host build (HostBuild path that produces ola_protoc_plugin)
failed in clean buildbot environments with:

  configure: error: Missing the uuid library

even though libuuid is provided by tools/util-linux and is staged
at $(STAGING_DIR_HOST)/lib/pkgconfig/uuid.pc.

Root cause: HOST_CONFIGURE_VARS overrode PKG_CONFIG_PATH with

  PKG_CONFIG_PATH="$(STAGING_DIR_HOSTPKG)/usr/protobuf-compat/lib/\
pkgconfig:$(PKG_CONFIG_PATH)"

$(PKG_CONFIG_PATH) here is the make-time variable, which is never
set for host context (include/host-build.mk only exports it as a
per-rule env variable, not a make variable). It therefore expanded
to the empty string, and the resulting single-entry PKG_CONFIG_PATH
overrode the exported defaults from host-build.mk, hiding uuid.pc
from the host pkg-config wrapper.

Fix: refer to the env variable via shell expansion ($$PKG_CONFIG_PATH)
instead of the make variable. The make recipe expands $$ to $, and
the shell resolves $PKG_CONFIG_PATH from the inherited environment
populated by include/host-build.mk (which sets it to
$(STAGING_DIR_HOST)/lib/pkgconfig:$(HOST_BUILD_PREFIX)/lib/pkgconfig).

Fixes: 5e4f937e ("ola: use protobuf-compat instead of protobuf")
Signed-off-by: Alexandru Ardelean <redacted>