meson.build: drop explicit install:true from configure_file

meson implicitly sets install to whether install_dir is nonzero. Which means
it's superfluous anyway and removing it drops the meson warning:

WARNING: Project specifies a minimum meson_version '>= 0.41.0' but uses
features which were added in newer versions:
 * 0.50.0: {'install arg in configure_file'}

Fixes #334

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>