Hi Rasmus,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on linus/master]
[cannot apply to v5.3-rc6 next-20190830]
[if your patch is applied to the wrong git tree, please drop us a note to help improve the system]

url:    https://github.com/0day-ci/linux/commits/Rasmus-Villemoes/printf-add-support-for-printing-symbolic-error-codes/20190831-162013
config: x86_64-randconfig-b002-201934 (attached as .config)
compiler: gcc-7 (Debian 7.4.0-11) 7.4.0
reproduce:
        # save the attached .config to linux build tree
        make ARCH=x86_64 

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   In file included from <command-line>:0:0:
   include/linux/errcode.h: In function 'errcode':
>> include/linux/errcode.h:10:9: error: 'NULL' undeclared (first use in this function)
     return NULL;
            ^~~~
   include/linux/errcode.h:10:9: note: each undeclared identifier is reported only once for each function it appears in

vim +/NULL +10 include/linux/errcode.h

     4	
     5	#ifdef CONFIG_SYMBOLIC_ERRCODE
     6	const char *errcode(int code);
     7	#else
     8	static inline const char *errcode(int code)
     9	{
  > 10		return NULL;
    11	}
    12	#endif
    13	

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