selftests: safesetid: Move link library to LDLIBS

Move -lcap to LDLIBS from CFLAGS because it is a library
to be linked.

Without this, safesetid failed to build with link error
as below.

----
/usr/bin/ld: /tmp/ccL8rZHT.o: in function `drop_caps':
safesetid-test.c:(.text+0xe7): undefined reference to `cap_get_proc'
/usr/bin/ld: safesetid-test.c:(.text+0x107): undefined reference to `cap_set_flag'
/usr/bin/ld: safesetid-test.c:(.text+0x10f): undefined reference to `cap_set_proc'
/usr/bin/ld: safesetid-test.c:(.text+0x117): undefined reference to `cap_free'
/usr/bin/ld: safesetid-test.c:(.text+0x136): undefined reference to `cap_clear'
collect2: error: ld returned 1 exit status
----

Fixes: c67e8ec0 ("LSM: SafeSetID: add selftest")
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/safesetid/Makefile

 # SPDX-License-Identifier: GPL-2.0
 # Makefile for mount selftests.
-CFLAGS = -Wall -lcap -O2
+CFLAGS = -Wall -O2
+LDLIBS = -lcap
 
 TEST_PROGS := run_tests.sh
 TEST_GEN_FILES := safesetid-test