package/pipewire: link examples with -latomic

Link examples with -latomic to avoid the following build failure raised
since commit 3edc1cc4:

FAILED: spa/examples/adapter-control
/home/buildroot/autobuild/instance-0/output-1/host/bin/sparc-linux-gcc  -o spa/examples/adapter-control spa/examples/adapter-control.p/adapter-control.c.o -Wl,--as-needed -Wl,--no-undefined -Wl,-O1 -Wl,--start-group -lintl -ldl -pthread -lm -Wl,--end-group
/home/buildroot/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc-buildroot-linux-uclibc/10.3.0/../../../../sparc-buildroot-linux-uclibc/bin/ld: spa/examples/adapter-control.p/adapter-control.c.o: in function `spa_graph_node_trigger':
adapter-control.c:(.text+0xdf4): undefined reference to `__atomic_fetch_sub_4'

Fixes:
 - http://autobuild.buildroot.org/results/192f40a5c6e05fc11507494e19db52a47082fc35



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>