include linux/stddef.h in swab.h uapi header

swab.h uses __always_inline without including the header where it is defined, this is exposed by musl based distributions where this macro is not defined by system C library headers unlike glibc where it is defined in sys/cdefs.h and that header gets pulled in indirectly via

features.h -> sys/cdefs.h

and features.h gets pulled in a lot of headers. Therefore it may work in cases where features.h is includes but not otherwise.

Adding linux/stddef.h here ensures that __always_inline is always defined independent of which C library is used in userspace

---
 include/uapi/linux/swab.h | 1 +
 1 file changed, 1 insertion(+)

index 23cd84868cc3..acddbe50a20d 100644
@@ -3,6 +3,7 @@
 #define _UAPI_LINUX_SWAB_H
 
 #include <linux/types.h>
+#include <linux/stddef.h>
 #include <linux/compiler.h>
 #include <asm/swab.h>
 

2.20.0