Install shared libraries in multiarch locations

This should in principle allow loading plugins (which are executables)
from foreign architectures, although it will not allow loading foreign
modules (which are dlopen'ed).

I'm deliberately not changing the ${libdir} at this stage, because gimp
seems to treat the plugin directories derived from the ${libdir} as
configuration parameters that might be saved in user configuration, so
modifying plugin-loading code to look in /usr/lib in addition to
/usr/lib/${multiarch} will not necessarily be effective.

Closes: #737725

debian/libgimp2.0-dev.install

@@ -1,4 +1,4 @@
-usr/lib/*.so
+usr/lib/*.so usr/lib/${DEB_HOST_MULTIARCH}
 usr/lib/pkgconfig/*.pc usr/lib/${DEB_HOST_MULTIARCH}/pkgconfig
 usr/share/man/man1/gimptool-?.?.1
 usr/share/aclocal/gimp-?.?.m4

debian/libgimp2.0.install

@@ -1,2 +1,2 @@
-usr/lib/lib*.so.*
+usr/lib/lib*.so.* usr/lib/${DEB_HOST_MULTIARCH}
 usr/share/locale/*/LC_MESSAGES/*-libgimp.mo