selftests: pidfd: Fix undefined reference to pthread_create()

Fix build failure:

undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status

Fix CFLAGS to include pthread correctly.

Fixes: 740378d ("pidfd: add polling selftests")
Signed-off-by: Shuah Khan <[email protected]>
Reviewed-by: Christian Brauner <[email protected]>
Cc: <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: shuahkh

tools/testing/selftests/pidfd/Makefile

@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0-only
-CFLAGS += -g -I../../../../usr/include/ -lpthread
+CFLAGS += -g -I../../../../usr/include/ -pthread
 
 TEST_GEN_PROGS := pidfd_test pidfd_open_test pidfd_poll_test pidfd_wait