xbps-src: show-build-deps: take only_for_archs into account.

Silently error out if arch is not supported... my new bulk build code
expects ERROR if pkg cannot be built for the target arch.

common/xbps-src/shutils/show.sh

@@ -50,6 +50,8 @@ show_pkg_files() {
 show_pkg_build_deps() {
     local f=
 
+    BEGIN_INSTALL=1
+    check_pkg_arch $XBPS_CROSS_BUILD
     # build time deps
     for f in ${hostmakedepends} ${makedepends}; do
         echo "$f"