package/gdk-pixbuf: fix test compilation (don't build the test suite)

Not needed + breaks compilation on some platforms.
The compilation error was seen on 2.42.8, but also relevant
for 2.42.9 to not compile the test suite.

The option was added in 2.42.9:
https://github.com/GNOME/gdk-pixbuf/commit/801eef111df624f4377baed9a90c94b6a2d4340c

```
[179/246] Generating tests/resources.c with a custom command
FAILED: tests/resources.c
/buildroot/output/host/bin/python3 /buildroot/output/build/host-gdk-pixbuf-2.42.8/build-aux/gen-resources.py --glib-compile-resources=/buildroot/output/host/bin/glib-compile-resources --pixdata=/buildroot/output/build/host-gdk-pixbuf-2.42.8/build/gdk-pixbuf/gdk-pixbuf-pixdata --loaders=/buildroot/output/build/host-gdk-pixbuf-2.42.8/build/gdk-pixbuf/loaders.cache --sourcedir=/buildroot/output/build/host-gdk-pixbuf-2.42.8/tests --source ../tests/resources.gresource.xml tests/resources.c
failed to load "/buildroot/output/build/host-gdk-pixbuf-2.42.8/tests/icc-profile.png": Couldn?t recognize the image file format for file ?/buildroot/output/build/host-gdk-pixbuf-2.42.8/tests/icc-profile.png?
../tests/resources.gresource.xml: Child process exited with code 1.
```

Signed-off-by: Thomas Devoogdt <thomas.devoogdt@barco.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/gdk-pixbuf/gdk-pixbuf.mk

@@ -19,11 +19,13 @@ HOST_GDK_PIXBUF_DEPENDENCIES = host-libpng host-pkgconf host-libglib2
 
 GDK_PIXBUF_CONF_OPTS = \
 	-Dgio_sniffing=false \
+	-Dtests=false \
 	-Dinstalled_tests=false \
 	-Dman=false
 
 HOST_GDK_PIXBUF_CONF_OPTS = \
 	-Dgio_sniffing=false \
+	-Dtests=false \
 	-Dinstalled_tests=false \
 	-Dintrospection=disabled \
 	-Drelocatable=true \