community/prometheus: fix provided ldflags

`-extldflags` is used to provide LDFLAGS to the external linker.

159018314b (community/prometheus: fix provided ldflags, 2020-09-19)
extended LDFLAGS with flags that are meant for Go itself. As long as
`$LDLFAGS` is set to a value this works.

But when `$LDFLAGS` is empty, it would pass the flags ment for Go to the
external linker, which will then error with a usage message.

Fix this by only supply -extldflags when `$LDFLAGS` is not empty.
7 jobs for !12641 with prometheus-fix-build-issue in 7 minutes and 57 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Lint
failed #210409
x86_64 docker-alpine allowed to fail
lint

00:00:21

 
  Build
passed #210414
aarch64 ci-build docker-alpine
build-aarch64

00:06:02

passed #210415
armv7 ci-build docker-alpine
build-armv7

00:07:33

passed #210413
ppc64le ci-build docker-alpine
build-ppc64le

00:07:16

passed #210412
s390x ci-build docker-alpine
build-s390x

00:03:09

passed #210411
x86 ci-build docker-alpine
build-x86

00:04:17

passed #210410
x86_64 ci-build docker-alpine
build-x86_64

00:03:08

 
Name Stage Failure
failed
lint Lint
                  apkbuild-lint:
======================================================


IC:[AL7]:APKBUILD:32:indent with tabs
IC:[AL7]:APKBUILD:34:indent with tabs

Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1