glib: install poppler-glib-demo when built
When building poppler-glib-demo
(BUILD_GTK_TESTS
option)
let's also install it, otherwise it can only be accessed
from the build directory.
When using JHBuild to build Poppler, this commit makes
jhbuild run poppler-glib-demo some_file.pdf
to actually
work, as the binary gets installed to the corresponding /bin
directory.