sh: Define missing __NR_readahead.

For some reason sh was missing __NR_readahead, even though the
syscall was wired up, and the slot was reserved. Caught with
dwmw2's missing syscall checker.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

include/asm-sh/unistd.h

@@ -233,6 +233,7 @@
 #define __NR_fcntl64		221
 /* 223 is unused */
 #define __NR_gettid		224
+#define __NR_readahead		225
 #define __NR_setxattr		226
 #define __NR_lsetxattr		227
 #define __NR_fsetxattr		228