vabackend: make NVCodec struct aligned

Add the `__attribute__((aligned))` attribute to the `NVCodec`
type so that it has "the maximum alignment for the target".
This is needed because these structs are placed into a
section of the executable and the linker will align
the objects to e.g. 16-byte boundaries on x86-64 regardless
of the actual size of the object.

Currently, the size of this struct is just right, so this
is technically not needed, but should it be extended in the
future, this will prevent surprises.

src/vabackend.h

@@ -126,13 +126,15 @@ typedef cudaVideoCodec (*ComputeCudaCodec)(VAProfile);
 
 //padding/alignment is very important to this structure as it's placed in it's own section
 //in the executable.
-typedef struct _NVCodec
+struct _NVCodec
 {
     ComputeCudaCodec    computeCudaCodec;
     HandlerFunc         handlers[VABufferTypeMax];
     int                 supportedProfileCount;
     const VAProfile     *supportedProfiles;
-} NVCodec;
+} __attribute__((aligned));
+
+typedef struct _NVCodec NVCodec;
 
 void appendBuffer(AppendableBuffer *ab, const void *buf, uint64_t size);
 int pictureIdxFromSurfaceId(NVDriver *ctx, VASurfaceID surf);