weston-ini-conf: explicitly enable compositor gRPC proxy

Add "grpc-proxy" to the new WESTON_FRAGMENTS_BASE to explicitly
enable agl-shell-grpc-proxy, thus matching the configuration the
agl-compositor bbappend in this layer enables.

Bug-AGL: SPEC-5183

Change-Id: Id842f3570ef6faf5c6424f201dbec1f896986378
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl-demo/+/30005
Tested-by: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
ci-image-build: Jenkins Job builder account