Make error on Debian 11 with Go

[dr1nf3rn0]

Fixes #30 issue

---

Fixes error occuring when making project from master branch:

[gopher@HYPERION pgpool2_exporter]$ make
>> building binaries
 >   pgpool2_exporter
# github.com/pgpool/pgpool2_exporter
./pgpool2_exporter.go:31:2: "os" imported and not used
!! command failed: build -o /home/gopher/pgpool2_exporter/pgpool2_exporter -ldflags -X github.com/prometheus/common/version.Version=1.2.1 -X github.com/prometheus/common/version.Revision=c76acd4012c52869f11c3fffd7ab626e2d0144b5 -X github.com/prometheus/common/version.Branch=master -X github.com/prometheus/common/version.BuildUser=gopher@HYPERION -X github.com/prometheus/common/version.BuildDate=20240111-15:05:24  -extldflags '-static' -a -tags 'netgo static_build' github.com/pgpool/pgpool2_exporter/cmd/pgpool2_exporter: exit status 1
make: *** [Makefile:17: build] Error 1

[exfly]

dup #32