hwcontext_opencl: include header file in HEADERS

This matches the inclusion of the other hwcontext_<hwaccel>.h headers.
The skipping of the header depending on build flags is already present.
Signed-off-by: Daniel Playfair Cal: <daniel.playfair.cal@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavutil/Makefile

@@ -40,6 +40,7 @@ HEADERS = adler32.h                                                     \
           hwcontext_dxva2.h                                             \
           hwcontext_qsv.h                                               \
           hwcontext_mediacodec.h                                        \
+          hwcontext_opencl.h                                            \
           hwcontext_vaapi.h                                             \
           hwcontext_videotoolbox.h                                      \
           hwcontext_vdpau.h                                             \