fix for gitlab ci

2 jobs for gitlab-ci in 11 minutes and 52 seconds (queued for 3 seconds)
Name Stage Failure
failed
alpine Buildandtest
--- FAIL: Test (3.90s)
FAIL
coverage: 97.9% of statements
FAIL github.com/otrv4/otrng-prekey-server 3.905s
# runtime/cgo
exec: "gcc": executable file not found in $PATH
FAIL github.com/otrv4/otrng-prekey-server/server/raw [build failed]
make: *** [Makefile:11: test] Error 2
ERROR: Job failed: exit code 2
failed
debian Buildandtest
	/go/pkg/mod/gopkg.in/check.v1@v1.0.0-20180628173108-788fd7840127/check.go:781 +0x628
gopkg.in/check%2ev1.(*suiteRunner).forkCall.func1(0xc0000b4100, 0xc0000c10e0, 0xc000118360)
/go/pkg/mod/gopkg.in/check.v1@v1.0.0-20180628173108-788fd7840127/check.go:675 +0x98
created by gopkg.in/check%2ev1.(*suiteRunner).forkCall
/go/pkg/mod/gopkg.in/check.v1@v1.0.0-20180628173108-788fd7840127/check.go:672 +0x1fb
FAIL github.com/otrv4/otrng-prekey-server/server/raw 600.016s
FAIL
make: *** [Makefile:11: test] Error 1
ERROR: Job failed: exit code 1