build.package: makedeps warning only for c-n2

Splitting makedepends in _host and _build only makes a difference when
building with cross-native2, as these packages end up in different
chroots.

For QEMU only and crossdirect we install all makedepends into the
foreign chroot. For cross-native (v1) we install everything into the
native chroot.

Show the warning only for cross-native2 so people don't think they have
to adjust their APKBUILDs here. Most of Alpine's APKBUILDs currently
don't have this split either.

Part-of: !2571

pmb/build/_package.py

@@ -681,7 +681,7 @@ def packages(
             depends_build = apkbuild["makedepends_build"]
         else:
             depends_build = apkbuild["makedepends"]
-            if depends_build and depends_host:
+            if depends_build and depends_host and cross == CrossCompile.CROSS_NATIVE2:
                 logging.warning(
                     "WARNING: makedepends not split into _host and _build variants."
                     " Trying to install all makedepends in both environments, please"