build: Add -lrt to AM_LDFLAGS

This is needed to link programs using clock_gettime and related
functions when compiling with older glibc versions.

This should fix the Jenkins build. Nevertheless fixing this in
configure.ac were probably nicer.

Sponsored-by: On-Waves ehf
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 77760f3..a9fc9dd 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -1,4 +1,5 @@
 AM_CPPFLAGS = $(STD_DEFINES_AND_INCLUDES) $(LIBOSMOCORE_CFLAGS) $(LIBOSMOGB_CFLAGS) $(LIBOSMOGSM_CFLAGS) -I$(top_srcdir)/src/
+AM_LDFLAGS = -lrt
 
 check_PROGRAMS = rlcmac/RLCMACTest alloc/AllocTest tbf/TbfTest types/TypesTest ms/MsTest llist/LListTest
 noinst_PROGRAMS = emu/pcu_emu