tests/docker: Set the correct cross-PKG_CONFIG_PATH in the MXE images

tests/docker: Set the correct cross-PKG_CONFIG_PATH in the MXE images

This silents a bunch of warnings while compiling the Slirp objects:

 $ make
 [...]
   CC      slirp/src/tftp.o
 Package glib-2.0 was not found in the pkg-config search path.
 Perhaps you should add the directory containing `glib-2.0.pc'
 to the PKG_CONFIG_PATH environment variable
 No package 'glib-2.0' found
   CC      slirp/src/udp6.o
 Package glib-2.0 was not found in the pkg-config search path.
 Perhaps you should add the directory containing `glib-2.0.pc'
 to the PKG_CONFIG_PATH environment variable
 No package 'glib-2.0' found
 [...]

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190715174817.18981-5-philmd@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>