Speculatively fix build bot

This should fix the issue found by:
https://lab.llvm.org/buildbot/#/builders/37/builds/12606

compiler-rt/test/safestack/lto.c

@@ -4,6 +4,7 @@
 // RUN: %run %t
 
 // Test that safe stack works with LTO.
+int puts(const char *);
 
 int main() {
   char c[] = "hello world";