ARM: nommu: fix warning with checksyscalls.sh

  CALL    scripts/checksyscalls.sh
<stdin>:46:1: warning: "__IGNORE_migrate_pages" redefined
In file included from <stdin>:2:
arch/arm/include/asm/unistd.h:482:1: warning: this is the location of the previous definition

This is caused because we define __IGNORE_migrate_pages to be 1, but
in the case of nommu, it's defined to be empty.  Fix this by just
defining the __IGNORE_ symbols to be empty.
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/unistd.h

@@ -478,8 +478,8 @@
 /*
  * Unimplemented (or alternatively implemented) syscalls
  */
-#define __IGNORE_fadvise64_64		1
-#define __IGNORE_migrate_pages		1
+#define __IGNORE_fadvise64_64
+#define __IGNORE_migrate_pages
 
 #endif /* __KERNEL__ */
 #endif /* __ASM_ARM_UNISTD_H */