hb-private.hh: Add fallback implementation for round()

Add a simplistic round() implementation for our purposes, used when the
compiler does not support round() directly.

src/hb-private.hh

diff --git a/src/hb-private.hh b/src/hb-private.hh
index daa496e..62a103c 100644
--- a/src/hb-private.hh
+++ b/src/hb-private.hh
@@ -1070,4 +1070,17 @@
 };
 
 
+/* fallback for round() */
+#if !defined (HAVE_ROUND) && !defined (HAVE_DECL_ROUND)
+static inline double
+round (double x)
+{
+  if (x >= 0)
+    return floor (x + 0.5);
+  else
+    return ceil (x - 0.5);
+}
+#endif
+
+
 #endif /* HB_PRIVATE_HH */