meson: use boolean value for boolean options

string values for boolean options will be deprecated in a future version
of meson.

Signed-off-by: Sertonix <sertonix@posteo.net>

meson_options.txt

@@ -45,7 +45,7 @@ option(
   'require_userns',
   type : 'boolean',
   description : 'require user namespaces by default when installed setuid',
-  value : 'false',
+  value : false,
 )
 option(
   'selinux',
@@ -57,7 +57,7 @@ option(
   'tests',
   type : 'boolean',
   description : 'build tests',
-  value : 'true',
+  value : true,
 )
 option(
   'zsh_completion',