meson: fix source file inclusion in libaegisub on osx

libaegisub/meson.build

@@ -58,6 +58,8 @@ if host_machine.system() == 'darwin'
     ]
 else
     libaegisub_src += 'common/dispatch.cpp'
+endif
+
 if host_machine.system() == 'windows'
     libaegisub_src += [
         'windows/access.cpp',
@@ -76,7 +78,6 @@ else
         'unix/util.cpp',
     ]
 endif
-endif
 
 libaegisub_cpp_pch = ['include/lagi_pre.h']
 libaegisub_c_pch = ['include/lagi_pre_c.h']