Fix build with unreleased FLAC 1.3.x

The issue was initialy reported here:
http://bugzilla.redhat.com/891123

FLAC upstream has always expected to include <FLAC/ suffix
for included headers as shown in examples/c/decode/file/main.c

The FLAC suffix will not be be made available in the flac.pc
for the next release:
http://lists.xiph.org/pipermail/flac-dev/2012-April/003355.htmlSigned-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/flac.c

@@ -37,8 +37,8 @@
 #include <vlc_plugin.h>
 #include <vlc_codec.h>
 
-#include <stream_decoder.h>
-#include <stream_encoder.h>
+#include <FLAC/stream_decoder.h>
+#include <FLAC/stream_encoder.h>
 
 #include <vlc_block_helper.h>
 #include <vlc_bits.h>