[Support] Fix Wundef warning

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368984 91177308-0d34-0410-b5e6-96231b3b80d8

Author: svenvh

lib/Support/Signposts.cpp

@@ -78,6 +78,8 @@ class SignpostEmitterImpl {
 
 #if LLVM_SUPPORT_XCODE_SIGNPOSTS
 #define HAVE_ANY_SIGNPOST_IMPL 1
+#else
+#define HAVE_ANY_SIGNPOST_IMPL 0
 #endif
 
 SignpostEmitter::SignpostEmitter() {