testing/marxan: remove redundant makedepends

`gcc` is part of `build-base`: no need to include it as `makedepends`.
8 jobs for !22427 with package/testing/marxan/initial-packaging in 2 minutes and 15 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Lint
passed #418521
docker-alpine x86_64
lint

00:00:20

 
  Build
passed #418526
docker-alpine ci-build aarch64
build-aarch64

00:01:24

passed #418528
docker-alpine ci-build armhf
build-armhf

00:01:52

passed #418527
docker-alpine ci-build armv7
build-armv7

00:01:32

passed #418525
docker-alpine ci-build ppc64le
build-ppc64le

00:01:45

passed #418524
docker-alpine ci-build s390x
build-s390x

00:00:54

passed #418523
docker-alpine ci-build x86
build-x86

00:01:00

passed #418522
docker-alpine ci-build x86_64
build-x86_64

00:00:49