egl: fix line continuation

egl: fix line continuation

Trailing space after the backslash meant the rest of the AM_CFLAGS lines
were no longer included.
This has been silently ignored because of the next line starting with
a `-` dash, instructing make to be silent about that line.

Fixes: 02cc359372773800de81 "egl/wayland: Use linux-dmabuf interface for buffers"
Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>