avcodec/dds: Fix runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

Fixes: 1380/clusterfuzz-testcase-minimized-650122545122508

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/dds.c

@@ -39,7 +39,7 @@
 
 #define DDPF_FOURCC    (1 <<  2)
 #define DDPF_PALETTE   (1 <<  5)
-#define DDPF_NORMALMAP (1 << 31)
+#define DDPF_NORMALMAP (1U << 31)
 
 enum DDSPostProc {
     DDS_NONE = 0,