v4l2: v4l2_compat: Selectively disable -Wmissing-declarations

_FORTIFY_SOURCE redirects the open*() calls to __open*_2() functions.
The libcamera V4L2 adaptation layer intercepts those functions to
support applications compiled with _FORTIFY_SOURCE. When _FORTIFY_SOURCE
is not enabled, the C library headers will not provide declarations for
the fortified functions, which will cause missing declaration warnings
when we unable them.

Fix this by disabling the -Wmissing-declarations warnings selectively
for the _FORTIFY_SOURCE functions. To avoid sparkling pragmas around,
move the relevant function definitions next to each other.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>