package/ghostscript: fix xlib dependency

Since the package bump to v10.05.0 the xlib libraries detection has been
improved to error missing x libraries (see [2]).

The following error is happening on the autobuilder when building
ghostscript with xlib_libX11:

```
checking for XOpenDisplay in -lX11... yes
checking for XdbeQueryExtension in -lXext... yes
checking for XtAppCreateShell in -lXt... no
configure: error: X11 libraries (libX11, libXt, libXext) not available, either install them, or rerun configure with "--without-x"
make: *** [package/pkg-generic.mk:263: /home/buildroot/instance-0/output-1/build/ghostscript-10.05.1/.stamp_configured] Error 1
```

This error can be reproduced with the following config:

```
cat <<EOF >.config
BR2_arm=y
BR2_cortex_a7=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_BOOTLIN=y
BR2_PACKAGE_XORG7=y
BR2_PACKAGE_XLIB_LIBX11=y
BR2_PACKAGE_GHOSTSCRIPT=y
EOF
make olddefconfig
make
```

This patch fixes the xlib dependencies of the ghostscript package to
correctly build with X support:

- The 'XtAppCreateShell' function is provided by the package
  'xlib_libXt'.
- The 'XdbeQueryExtension' function is provided by the package
  'xlib_libXext'.

They both select the 'xlib_libX11' package.

[1] 9abf662cfd package/ghostscript: security bump to version 10.05.0
[2] https://github.com/ArtifexSoftware/ghostpdl/commit/37eede31e12814fae69744e7842a5640004e5a65

Fixes: https://autobuild.buildroot.org/results/45d/45d0414f341feb2354478b145396fe137f9cc6a6
Signed-off-by: Thomas Perale <thomas.perale@mind.be>
Signed-off-by: Romain Naour <romain.naour@smile.fr>

package/ghostscript/ghostscript.mk

@@ -85,8 +85,8 @@ else
 GHOSTSCRIPT_CONF_OPTS += --disable-cups
 endif
 
-ifeq ($(BR2_PACKAGE_XLIB_LIBX11),y)
-GHOSTSCRIPT_DEPENDENCIES += xlib_libX11
+ifeq ($(BR2_PACKAGE_XLIB_LIBXT)$(BR2_PACKAGE_XLIB_LIBXEXT),yy)
+GHOSTSCRIPT_DEPENDENCIES += xlib_libXt xlib_libXext
 GHOSTSCRIPT_CONF_OPTS += --with-x
 else
 GHOSTSCRIPT_CONF_OPTS += --without-x