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 <ricardo.canuelo@collabora.com>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/futex/functional/Makefile

@@ -3,11 +3,11 @@ INCLUDES := -I../include -I../../ -I../../../../../usr/include/
 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 \
@@ -24,5 +24,3 @@ TEST_PROGS := run.sh
 top_srcdir = ../../../../..
 DEFAULT_INSTALL_HDR_PATH := 1
 include ../../lib.mk
-
-$(TEST_GEN_FILES): $(HEADERS)