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

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: 6989310f ("wireless: Use offsetof instead of custom macro.")
Signed-off-by: NHauke Mehrtens <hauke@hauke-m.de>
Link: https://lore.kernel.org/r/20200521201422.16493-1-hauke@hauke-m.deSigned-off-by: NJohannes Berg <johannes.berg@intel.com>

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 *****************************/
 /*