No librt on OpenBSD

Index: meson.build
--- meson.build.orig
+++ meson.build
@@ -121,7 +121,6 @@ pixman = dependency('pixman-1',
 	default_options: ['werror=false'],
 )
 math = cc.find_library('m')
-rt = cc.find_library('rt')
 
 wlr_files = []
 wlr_deps = [
@@ -130,7 +129,6 @@ wlr_deps = [
 	xkbcommon,
 	pixman,
 	math,
-	rt,
 ]
 
 subdir('protocol')
