sandbox: solve undefined reference to pthread_kill symbol

sandbox: solve undefined reference to pthread_kill symbol

This patch is to solve the sandbox building error:
$ make O=build-sandbox -s sandbox_defconfig
$ make O=build-sandbox -s -j2
/usr/bin/ld: /tmp/u-boot.27rzOu.ltrans58.ltrans.o: undefined reference to symbol 'pthread_kill@@GLIBC_2.2.5'
/usr/bin/ld: /lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
[...]

Fixes: b989f9ed9fe1 ("test: lib: add initjmp() test")
Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Tested-by: Patrice Chotard <patrice.chotard@foss.st.com>