src/libcamera/meson.build: link with atomic when needed

On some architectures, atomic binutils are provided by the libatomic
library from gcc. Linking with libatomic is therefore necessary,
otherwise the build fails with:

src/libcamera/4ab8042@@camera@sha/message.cpp.o: In function `libcamera::Message::registerMessageType()':
message.cpp:(.text+0x178): undefined reference to `__atomic_fetch_add_4'
collect2: error: ld returned 1 exit status

This is often for example the case on sparc v8 32 bits.

Fixes:
 - http://autobuild.buildroot.org/results/1f0b8338f5f39aa86b9d432598dae2f53c5f7c84

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
[Kieran: Updated commit message to refer to build failure on current
master, rather than the old code currently built by buildroot]
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/meson.build

@@ -99,6 +99,7 @@ version_cpp = vcs_tag(command : [gen_version, meson.build_root()],
 libcamera_sources += version_cpp
 
 libcamera_deps = [
+    cc.find_library('atomic', required: false),
     cc.find_library('dl'),
     libudev,
     dependency('threads'),