package/mpv: link with libatomic if needed

Fixes:
 - http://autobuild.buildroot.org/results/12f4580429427b6b546184366f74da16f83d692c

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 71cb0f0f722875100cd18df9484a7a30f53bfbeb)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/mpv/mpv.mk

@@ -235,4 +235,8 @@ else
 MPV_CONF_OPTS += --disable-x11
 endif
 
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+MPV_CONF_ENV += LDFLAGS="$(TARGET_LDFLAGS) -latomic"
+endif
+
 $(eval $(waf-package))