build-sys: link test-seccomp against seccomp libs (#4560)

Fixes build error on recent toolchains:

  ../src/test/test-seccomp.c:35: error: undefined reference to 'seccomp_arch_native'
  collect2: error: ld returned 1 exit status

Makefile.am

@@ -2038,7 +2038,8 @@ test_seccomp_SOURCES = \
 	src/test/test-seccomp.c
 
 test_seccomp_LDADD = \
-	libsystemd-shared.la
+	libsystemd-shared.la \
+	$(SECCOMP_LIBS)
 
 test_namespace_LDADD = \
 	libcore.la