[gnote/wip/hadess/build-failures: 1/3] Fix AbstractAddin compilation error

[Bastien Nocera <hadess src gnome org>]

commit b98be7f658fe45c11cedc42ecd8d5a41cd17041b
Author: Bastien Nocera <hadess hadess net>
Date:   Mon Mar 22 15:04:01 2021 +0100

    Fix AbstractAddin compilation error
    
    gnote/src/abstractaddin.cpp: In constructor ‘gnote::AbstractAddin::AbstractAddin()’:
    gnote/src/abstractaddin.cpp:29:15: error: ‘NULL’ was not declared in this scope
       29 |     : m_gnote(NULL)
          |               ^~~~
    gnote/src/abstractaddin.cpp:25:1: note: ‘NULL’ is defined in header ‘<cstddef>’; did you forget to 
‘#include <cstddef>’?
       24 | #include "abstractaddin.hpp"
      +++ |+#include <cstddef>
       25 |

 src/abstractaddin.cpp | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/src/abstractaddin.cpp b/src/abstractaddin.cpp
index 764b5bde..209dddfb 100644
--- a/src/abstractaddin.cpp
+++ b/src/abstractaddin.cpp
@@ -22,6 +22,7 @@
 
 
 #include "abstractaddin.hpp"
+#include <cstddef>
 
 namespace gnote {