avdevice/alldevices: Mark devices as hidden

Both GCC and Clang create code that inlines the loops in
next_input() and next_output() at high optimization
levels (presumably when there are not too many devices)
and this code leads to the creation of .got entries:
  e7:	48 3b 3d 00 00 00 00 	cmp    0x0(%rip),%rdi        # ee <av_input_video_device_next+0xe>
			ea: R_X86_64_REX_GOTPCRELX	ff_alsa_demuxer-0x4
  ee:	74 43                	je     133 <av_input_video_device_next+0x53>
  f0:	48 3b 3d 00 00 00 00 	cmp    0x0(%rip),%rdi        # f7 <av_input_video_device_next+0x17>
			f3: R_X86_64_REX_GOTPCRELX	ff_fbdev_demuxer-0x4
  f7:	74 41                	je     13a <av_input_video_device_next+0x5a>

These relocations can't be fixed up lateron when it is known
that the symbols exist in the same DSO.

This commit therefore marks these symbols as hidden, leading
to code like this:
  f7:	48 8d 05 00 00 00 00 	lea    0x0(%rip),%rax        # fe <av_input_video_device_next+0xe>
			fa: R_X86_64_PC32	ff_alsa_demuxer-0x4
  fe:	48 39 c7             	cmp    %rax,%rdi
 101:	74 55                	je     158 <av_input_video_device_next+0x68>
 103:	48 8d 05 00 00 00 00 	lea    0x0(%rip),%rax        # 10a <av_input_video_device_next+0x1a>
			106: R_X86_64_PC32	ff_fbdev_demuxer-0x4
 10a:	48 39 c7             	cmp    %rax,%rdi
 10d:	74 50                	je     15f <av_input_video_device_next+0x6f>

(Note: It is actually strange that the compiler creates code
that tries to read the addresses from the .got given that the
addresses can be read directly from indev_list/outdev_list.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavdevice/alldevices.c

@@ -18,11 +18,13 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include "libavutil/attributes_internal.h"
 #include "libavformat/demux.h"
 #include "libavformat/internal.h"
 #include "libavformat/mux.h"
 #include "avdevice.h"
 
+FF_VISIBILITY_PUSH_HIDDEN
 /* devices */
 extern const FFInputFormat  ff_alsa_demuxer;
 extern const FFOutputFormat ff_alsa_muxer;
@@ -59,6 +61,7 @@ extern const FFOutputFormat ff_xv_muxer;
 /* external libraries */
 extern const FFInputFormat  ff_libcdio_demuxer;
 extern const FFInputFormat  ff_libdc1394_demuxer;
+FF_VISIBILITY_POP_HIDDEN
 
 #include "libavdevice/outdev_list.c"
 #include "libavdevice/indev_list.c"