meson: Use the not_found_dep constant

Previously in the build description, a not_found_dep was defined. Make
use of it.

Author: Tristan Partin <tristan@neon.tech>
Discussion: https://www.postgresql.org/message-id/flat/CSPIJVUDZFKX.3KHMOAVGF94RV%40c3po

Author: petere

meson.build

@@ -558,7 +558,7 @@ endif
 ###############################################################
 
 bonjouropt = get_option('bonjour')
-bonjour = dependency('', required : false)
+bonjour = not_found_dep
 if cc.check_header('dns_sd.h', required: bonjouropt,
     args: test_c_args, include_directories: postgres_inc) and \
    cc.has_function('DNSServiceRegister',