community/spdlog: define SPDLOG_FMT_EXTERNAL in tweakme.h

Why it is not set when one uses -DSPDLOG_FMT_EXTERNAL=ON is beyond me

fixes #11601
7 jobs for !8701 with spdlog in 19 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Lint
failed #132675
docker-alpine x86_64 allowed to fail
lint

00:00:09

 
  Build
failed #132680
docker-alpine ci-build aarch64
build-aarch64

00:00:07

failed #132681
docker-alpine ci-build armv7
build-armv7

00:00:06

failed #132679
docker-alpine ci-build ppc64le
build-ppc64le

00:00:09

failed #132678
docker-alpine ci-build s390x
build-s390x

00:00:05

failed #132677
docker-alpine ci-build x86
build-x86

00:00:06

failed #132676
docker-alpine ci-build x86_64
build-x86_64

00:00:06

 
Name Stage Failure
failed
build-s390x Build
No job log
failed
lint Lint
No job log
failed
build-armv7 Build
No job log
failed
build-aarch64 Build
No job log
failed
build-x86_64 Build
No job log
failed
build-x86 Build
No job log
failed
build-ppc64le Build
No job log