[MIPS64] Fix mips64 build.

Build was broken by:
https://android-review.googlesource.com/133834

Use <unistd.h> to get syscall().
Remove <asm/unistd.h>, it gets included through <sys/syscall.h>.

Change-Id: Id762f6dea5f9538c19b79cdd46deda978efd50fe

libc/arch-mips64/bionic/stat.cpp

@@ -29,7 +29,7 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/syscall.h>
-#include <asm/unistd.h>
+#include <unistd.h>
 
 struct kernel_stat {
  unsigned int st_dev;