ibrdtn-tools: fix linking problem when building statically

The problem looks like this:

/br/output/host/usr/mipsel-buildroot-linux-uclibc/sysroot/usr//lib/librt.a(timer_routines.os):
In function `__start_helper_thread':
timer_routines.c:(.text+0x2f4): undefined reference to `pthread_atfork'
collect2: error: ld returned 1 exit status

Adding -pthread to LDFLAGS fixes the problem.

Fixes:

  http://autobuild.buildroot.net/results/a87/a87a764d3a28f835ed8645923594f7023a662ae8/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>