selftests/futex: fix build for clang

Don't use the test-specific header files as source files to force a
target dependency, as clang will complain if more than one source file
is used for a compile command with a single '-o' flag.

Use the proper Makefile variables instead as defined in
tools/testing/selftests/lib.mk.

Signed-off-by: Ricardo Cañuelo <[e-mail]>
Reviewed-by: André Almeida <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/futex/functional/Makefile

···
 CFLAGS := $(CFLAGS) -g -O2 -Wall -D_GNU_SOURCE -pthread $(INCLUDES) $(KHDR_INCLUDES)
 LDLIBS := -lpthread -lrt
 
-HEADERS := \
+LOCAL_HDRS := \
 	../include/futextest.h \
 	../include/atomic.h \
 	../include/logging.h
-TEST_GEN_FILES := \
+TEST_GEN_PROGS := \
 	futex_wait_timeout \
 	futex_wait_wouldblock \
 	futex_requeue_pi \
···
 top_srcdir = ../../../../..
 DEFAULT_INSTALL_HDR_PATH := 1
 include ../../lib.mk
-
-$(TEST_GEN_FILES): $(HEADERS)