tests: run ldconfig after installing extra software

The docker file builds and installs software into /usr/local but does
not run ldconfig. As a result QEMU links to libvirglrenderer.so, but
then crashes in "make check" unable to find the library.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>

tests/docker/dockerfiles/debian-amd64.docker

@@ -36,5 +36,7 @@ RUN git clone https://github.com/luigirizzo/netmap.git /usr/src/netmap
 RUN cd /usr/src/netmap/LINUX && ./configure --no-drivers --no-apps --kernel-dir=$(ls -d /usr/src/linux-headers-*-amd64) && make install
 ENV QEMU_CONFIGURE_OPTS --enable-netmap
 
+RUN ldconfig
+
 # gcrypt
 ENV QEMU_CONFIGURE_OPTS $QEMU_CONFIGURE_OPTS --enable-gcrypt