build: bump soversion

According to [1] this should be done at each release with breaking ABI
changes.

[1]: https://gitlab.freedesktop.org/wlroots/wlroots/-/wikis/Core-contributor-guide#releasing-a-new-version

Fixes: 7360810f2e ("build: bump to version 0.16.0")

meson.build

@@ -15,7 +15,7 @@ project(
 # necessary for bugfix releases. Increasing soversion is required because
 # wlroots never guarantees ABI stability -- only API stability is guaranteed
 # between minor releases.
-soversion = 10
+soversion = 11
 
 little_endian = host_machine.endian() == 'little'
 big_endian = host_machine.endian() == 'big'