build: assert wlroots version at comptime

This will prevent people compiling river against the wrong wlroots
version and wondering why it crashes.

deps/zig-wlroots

@@ -1 +1 @@
-Subproject commit c5f759484ec2bb6afdee54053f5b0aaccf060131
+Subproject commit b270171f7bfe19eb15c1bb2b20121dd9e7ac32f7

river/main.zig

@@ -16,9 +16,17 @@
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
 
 const std = @import("std");
+const assert = std.debug.assert;
 const os = std.os;
+
 const wlr = @import("wlroots");
 
+// River requires wlroots 0.14
+comptime {
+    assert(wlr.version.major == 0);
+    assert(wlr.version.minor == 14);
+}
+
 const build_options = @import("build_options");
 
 const c = @import("c.zig");