meson.build: correct license to HPND

Best explanation: 12-ish years ago when I started libwacom, I copied the
license from some X11-related repository assuming it was MIT and went
with that. Turns out it's actually the HPND-sell-variant:
"Historical Permission Notice and Disclaimer - sell variant"
https://spdx.org/licenses/HPND-sell-variant.html

No real effects since it's not really that much different to MIT afaict
but let's make sure meson prints out the correct SPDX license
identifier.

Signed-off-by: Peter Hutterer <[email protected]>

meson.build

@@ -1,6 +1,6 @@
 project('libwacom', 'c',
 	version: '2.8.0',
-	license: 'MIT/Expat',
+	license: 'HPND',
 	default_options: [ 'c_std=gnu99', 'warning_level=2' ],
 	meson_version: '>= 0.51.0')