component/glib: put slapos python3 in PATH

On old debian (9) where python3 is python3.5 this component fails to
build with an error like:

    Configuring gdbus-example-objectmanager-visibility.h with command
    Running command: /opt/slapgrid/shared/glib/60e920f1feec2451d51bb344cfcad9ab/.build/glib-2.76.3/tools/gen-visibility-macros.py 2.0 visibility-macros GDBUS_OBJECT_MANAGER_EXAMPLE /opt/slapgrid/shared/glib/60e920f1feec2451d51bb344cfcad9ab/.build/glib-2.76.3/builddir/gio/tests/gdbus-object-manager-example/gdbus-example-objectmanager-visibility.h
    --- stdout ---
    --- stderr ---
      File "/opt/slapgrid/shared/glib/60e920f1feec2451d51bb344cfcad9ab/.build/glib-2.76.3/tools/gen-visibility-macros.py", line 37
        """
        ^
    SyntaxError: invalid syntax

This is because this gen-visibility-macros.py script is executable with
a shebang:

    #!/bin/env python3

for python 3 softwares, this is slapos python, because the python
section from component/defaults.cfg injects the slapos' python in PATH,
but for python 2 software, slapos' python 2 is injected in path and
`python3` resolves to system python, which in that case fails because
f-strings are SyntaxError but more generally this showed a dependency
to system python, but we can not rely on system python here.

component/glib/buildout.cfg

@@ -7,6 +7,7 @@ extends =
   ../patch/buildout.cfg
   ../pcre2/buildout.cfg
   ../perl/buildout.cfg
+  ../python3/buildout.cfg
   ../pkgconfig/buildout.cfg
   ../xz-utils/buildout.cfg
   ../zlib/buildout.cfg
@@ -27,7 +28,7 @@ make-binary = ninja -C builddir
 pkg_config_depends = ${pcre2:location}/lib/pkgconfig:${libffi:location}/lib/pkgconfig:${zlib:location}/lib/pkgconfig
 environment =
   PKG_CONFIG_PATH=${:pkg_config_depends}
-  PATH=${ninja:location}/bin:${pkgconfig:location}/bin:${patch:location}/bin:${perl:location}/bin:${xz-utils:location}/bin:%(PATH)s
+  PATH=${python3:location}/bin:${ninja:location}/bin:${pkgconfig:location}/bin:${patch:location}/bin:${perl:location}/bin:${xz-utils:location}/bin:%(PATH)s
   CFLAGS=-I${gettext:location}/include
   LDFLAGS=-L${gettext:location}/lib -lintl -Wl,-rpath=${gettext:location}/lib -Wl,-rpath=${libffi:location}/lib -Wl,-rpath=${zlib:location}/lib -Wl,-rpath=@@LOCATION@@/lib -Wl,-rpath=${pcre2:location}/lib
 post-install = rm %(location)s/bin/gtester-report