package/alljoyn: needs BR2_TOOLCHAIN_HAS_SYNC_4

package/alljoyn: needs BR2_TOOLCHAIN_HAS_SYNC_4

After applying a patch like [1], the build failed with the following
error:
liballjoyn_c.a(Message.o): In function `alljoyn_message_destroy':
Message.cc:(.text+0x74): undefined reference to `__sync_sub_and_fetch_4'
liballjoyn.so: undefined reference to `__sync_bool_compare_and_swap_4'
liballjoyn.so: undefined reference to `__sync_add_and_fetch_4'

So, add dependency on BR2_TOOLCHAIN_HAS_SYNC_4 which also disable the
alljoyn build on sparc.

[1] https://git.buildroot.org/buildroot/commit/?id=c5e96d8935016456bea342db170ae6a139a8470f

Fixes:
http://autobuild.buildroot.net/results/cea/cea62a7ed7f1eda6f817d8e27fd645942def25ec

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>