Add missing include statement

Resolve a build failure for user applications that include <sys/uio.h>.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Shreshth Srivastava <[email protected]>
Closes openzfs#17781
Closes openzfs#17814

Author: Shreshth3

lib/libspl/include/sys/uio.h

@@ -41,6 +41,7 @@
 #ifndef	_LIBSPL_SYS_UIO_H
 #define	_LIBSPL_SYS_UIO_H
 
+#include <sys/sysmacros.h>
 #include <sys/types.h>
 #include_next <sys/uio.h>