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.
$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.