selftests: filesystems/epoll: fix build error

epoll build fails to find pthread lib. Fix Makefile to use LDLIBS
instead of LDFLAGS. LDLIBS is the right flag to use here with -l
option when invoking ld.

gcc -I../../../../../usr/include/  -lpthread  epoll_wakeup_test.c  -o .../tools/testing/selftests/filesystems/epoll/epoll_wakeup_test
/usr/bin/ld: /tmp/ccaZvJUl.o: in function `kill_timeout':
epoll_wakeup_test.c:(.text+0x4dd): undefined reference to `pthread_kill'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x4f2): undefined reference to `pthread_kill'
/usr/bin/ld: /tmp/ccaZvJUl.o: in function `epoll9':
epoll_wakeup_test.c:(.text+0x6382): undefined reference to `pthread_create'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x64d2): undefined reference to `pthread_create'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x6626): undefined reference to `pthread_join'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x684c): undefined reference to `pthread_tryjoin_np'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x6864): undefined reference to `pthread_kill'
/usr/bin/ld: epoll_wakeup_test.c:(.text+0x6878): undefined reference to `pthread_join'
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/filesystems/epoll/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
 CFLAGS += -I../../../../../usr/include/
-LDFLAGS += -lpthread
+LDLIBS += -lpthread
 TEST_GEN_PROGS := epoll_wakeup_test
 
 include ../../lib.mk