meson: add manpage support

It seems like meson doesn't want you to string together targets
like make does, but wants it all in one step. So another little
shell script it is.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>
Acked-by: Petri Latvala <petri.latvala@intel.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Acked-by: Radoslaw Szwichtenberg <radoslaw.szwichtenberg@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/meson.build b/meson.build
index 39749a0..2b49a0d 100644
--- a/meson.build
+++ b/meson.build
@@ -120,3 +120,4 @@
 	subdir('assembler')
 	subdir('overlay')
 endif
+subdir('man')