Do not define LBR16* macros in png.c in Q8 and Q16 builds.

diff --git a/coders/png.c b/coders/png.c
index 51fdfc0..fb55ecb 100644
--- a/coders/png.c
+++ b/coders/png.c
@@ -501,6 +501,7 @@
         }
 
 
+#if MAGICKCORE_QUANTUM_DEPTH > 16
 /* LBR16: Replicate top 16 bits */
 
 #define LBR16PacketRed(pixelpacket) \
@@ -578,6 +579,7 @@
         LBR16PixelRGB((pixel)); \
         LBR16PixelAlpha((pixel)); \
         }
+#endif /* MAGICKCORE_QUANTUM_DEPTH > 16 */
 
 /*
   Establish thread safety.