compat: include linux/unistd.h within linux/compat.h

linux/compat.h does not include linux/unistd.h but the compat.h header
file contains various conditional

 #ifdef __ARCH_WANT_COMPAT_...
 asmlinkage long compat...()
 #endif

compat system call function declarations.
If linux/unistd.h isn't included it depends on previous includes if those
__ARCH_WANT_COMPAT_... defines are defined or not. So add an additional
linux/unistd.h include.

Should fix this compile error on tile:

include/uapi/asm-generic/unistd.h:195:1: error: 'compat_sys_getdents64' undeclared
make[3]: *** [arch/tile/kernel/compat.o] Error 1

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>