meson: fix header path of ossp-uuid

The ossp-uuid pkg-config file includes the necessary -I to include the header
as uuid.h. Previously this would only work if ossp-uuid has its headers in
ossp/ in an already searched path.

Reported-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/7bb8b8a5-5297-ab3c-3412-466fd0124d00@dunslane.net

meson.build

@@ -1268,7 +1268,7 @@ if uuidopt != 'none'
   elif uuidopt == 'ossp'
     uuid = dependency('ossp-uuid', required: true)
     uuidfunc = 'uuid_export'
-    uuidheader = 'ossp/uuid.h'
+    uuidheader = 'uuid.h'
   else
     error('huh')
   endif