Fix #pragma message for MSVC.

__LOC__ was renamed to _HB__LOC__ in cd7555 but the corresponding
change wasn't made in hb-private.h.

src/hb-private.h

diff --git a/src/hb-private.h b/src/hb-private.h
index 96b9464..9f4f8b7 100644
--- a/src/hb-private.h
+++ b/src/hb-private.h
@@ -211,7 +211,7 @@
 #else
 
 #ifdef _MSC_VER
-#pragma message(__LOC__"Could not find any system to define platform macros, library will NOT be thread-safe")
+#pragma message(_HB__LOC__"Could not find any system to define platform macros, library will NOT be thread-safe")
 #else
 #warning "Could not find any system to define platform macros, library will NOT be thread-safe"
 #endif