xbps-src: fix show_pkg_build_depends after af6e1a7e797995f.

af6e1a7e797995f introduces dependencies based in subpkgs,
but this makes "xbps-src show-build-deps" return cyclic
dependencies in some cases, i.e:

$ ./xbps-src show-build-deps kirigami2
cmake
extra-cmake-modules
kcoreaddons
kirigami2
qt5
$

This is because `kirigami2-devel` depends on `kirigami2`, thus
introducing a cycle.

Skip cycles completely, note that "show-build-deps" should not
be used to collect deps for cross compilation, because you can't
know what type of dependency it is: host or target.

common/xbps-src/shutils/show.sh

@@ -68,6 +68,8 @@ show_pkg_build_depends() {
 
     # build time deps
     for f in ${_deps}; do
+        # ignore dependency on itself
+        [[ $f == $sourcepkg ]] && continue
         if [ ! -f $XBPS_SRCPKGDIR/$f/template ]; then
             msg_error "$pkgver: dependency '$f' does not exist!\n"
         fi