build: install river-layout-v3 to $PREFIX/share/river

This makes it easier for other projects (i.e. everyone's layout
generators) to depend on the protocol xml without needing to vendor it.

The river-layout-v3 protocol should remain stable for the
forseeable future, hopefully forever. On the other hand, the current
river-control-unstable-v1 and river-status-unstable-v1 protocols
will be replaced as soon as we have time to implement better
versions. Therefore, let's not encourage usage of the protocols we
intend to remove.

 build.zig | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/build.zig b/build.zig
index d54bd28..1f5d04f 100644
--- a/build.zig
+++ b/build.zig
@@ -124,30 +124,23 @@ pub fn build(b: *zbs.Builder) !void {
         rivertile.install();
     }
 
+    b.installFile("protocol/river-layout-v3.xml", "share/river/river-layout-v3.xml");
+
     if (man_pages) {
         const scdoc_step = ScdocStep.create(b);
         try scdoc_step.install();
     }
 
     if (bash_completion) {
-        b.installFile(
-            "completions/bash/riverctl",
-            "share/bash-completion/completions/riverctl",
-        );
+        b.installFile("completions/bash/riverctl", "share/bash-completion/completions/riverctl");
     }
 
     if (zsh_completion) {
-        b.installFile(
-            "completions/zsh/_riverctl",
-            "share/zsh/site-functions/_riverctl",
-        );
+        b.installFile("completions/zsh/_riverctl", "share/zsh/site-functions/_riverctl");
     }
 
     if (fish_completion) {
-        b.installFile(
-            "completions/fish/riverctl.fish",
-            "share/fish/vendor_completions.d/riverctl.fish",
-        );
+        b.installFile("completions/fish/riverctl.fish", "share/fish/vendor_completions.d/riverctl.fish");
     }
 
     {