Wrap the #include of <stdbool.h> in an #ifndef __cplusplus.

This should fix the MSVC build.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191357 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/llvm-c/lto.h b/include/llvm-c/lto.h
index 40110fd..d960617 100644
--- a/include/llvm-c/lto.h
+++ b/include/llvm-c/lto.h
@@ -16,7 +16,9 @@
 #ifndef LLVM_C_LTO_H
 #define LLVM_C_LTO_H
 
+#ifndef __cplusplus
 #include <stdbool.h>
+#endif
 #include <stddef.h>
 #include <unistd.h>