wireless: Use linux/stddef.h instead of stddef.h

BugLink: https://bugs.launchpad.net/bugs/1908562

commit 1b9ae0c upstream.

When compiling inside the kernel include linux/stddef.h instead of
stddef.h. When I compile this header file in backports for power PC I
run into a conflict with ptrdiff_t. I was unable to reproduce this in
mainline kernel. I still would like to fix this problem in the kernel.

Fixes: 6989310 ("wireless: Use offsetof instead of custom macro.")
Signed-off-by: Hauke Mehrtens <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Johannes Berg <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Kamal Mostafa <[email protected]>
Signed-off-by: Ian May <[email protected]>

Author: hauke

include/uapi/linux/wireless.h

@@ -74,7 +74,11 @@
 #include <linux/socket.h>		/* for "struct sockaddr" et al	*/
 #include <linux/if.h>			/* for IFNAMSIZ and co... */
 
-#include <stddef.h>                     /* for offsetof */
+#ifdef __KERNEL__
+#	include <linux/stddef.h>	/* for offsetof */
+#else
+#	include <stddef.h>		/* for offsetof */
+#endif
 
 /***************************** VERSION *****************************/
 /*