[MIPS] Add missing header for syscall

[petarj]

Summary

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.

lib/builtins/clear_cache.c

 
 #if defined(__NetBSD__) && defined(__arm__)
   #include <machine/sysarch.h>
 #endif
 
 #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.
      */
     static void clear_mips_cache(const void* Addr, size_t Size) {
       asm volatile (
         ".set push\n"
         ".set noreorder\n"