[MIPS] Add missing header for syscall

The syscall function itself is declared in <unistd.h>
Compiling compiler-rt with -Werror-implicit-function-declaration will
cause a build failure. This change fixes it.

Differential Revision: http://reviews.llvm.org/D7949

llvm-svn: 230838

compiler-rt/lib/builtins/clear_cache.c

@@ -25,6 +25,7 @@
 #if defined(__mips__)
   #include <sys/cachectl.h>
   #include <sys/syscall.h>
+  #include <unistd.h>
   #if defined(__ANDROID__) && defined(__LP64__)
     /*
      * clear_mips_cache - Invalidates instruction cache for Mips.